FAQ: How Do I Zero My Weapon and How Do I Maintain It?

How do I zero my weapon?

When you buy your riflescope, a wonderful idea would be to attach it to the gun and VOILA! You’re ready to shoot!

Unfortunately, that fantasy is not the reality, and there are some steps that need to be taken to align the crosshairs of the scope with the muzzle of the gun.

What you need to zero in your weapon

  • Rifle
  • Rings
  • Scope
  • A Range or Appropriate Shooting Location with all the Proper Precautions
  • Sand Bags or Mechanical Rest
  • Two Boxes of Ammunition (at least)
  • Ear and Eye Protection

Step 1: You need to be familiar with your gun and scope.  Know where the windage and elevation knobs are on your scope.  The elevation knob is usually on the top, and the windage should be on the right.

  • If you have never mounted a scope to your gun and you’re unsure, PLEASE take it to a competent gunsmith and have them mount it.

Step 2: At the range, a common distance to zero is at 100 yards, but your may wish to select different depending on purpose or preference.

Step 3: You need to position your rifle in a secure set-up, whether it is on a table, with sandbags or a mechanical rest, or in the prone position.  This will enhance your ability to make a consistent shot.  You should avoid kneeling or standing without a table or some means of steadying the rifle securely.  The elimination of vibrations or wavering is absolutely essential in obtaining a good zero.  You want your set-up to be consistent each time.  A good, steady rest is the key for a consistent, repeatable zero. 

Step 4: Now, in order to make sure your first shot is on the paper and to save ammo, I suggest you boresight your rifle. TIME OUT! 

  • To boresight, remove the bolt from the rifle and make sure your rifle is secure on sandbags or on a mechanical rest.  Look through the rifle bore from the rear of the rifle.    Locate the target and place in the middle of the bore.  You will want to move the rifle until you can see the target centered in the field of view through the barrel.  Once you have done that, do not move the rifle, but adjust the scope, using the elevation and windage knobs accordingly to align the crosshairs on the target.

Step 5: Once you’ve sighted in, reinsert the bolt and get the rifle in a steady shooting position. 

  • Recommendation: Dry fire now! Carefully sight in the target and very gently squeeze the trigger. Practice 5-10 times to get acquainted with the trigger for a smooth, rather than jerky pull.

Step 6: Now you’re ready to affirm the boresighted zero.  Load the rifle with one shell and check your shooting position.  Make sure the rifle is solidly mounted on a rest, and adjust your shoulder and cheek on the gun with adequate pressure.  Line the crosshairs of the scope on the center of the target. Now, gently press the trigger for the shot.

  • Here you will want to fire three shots, to make sure the grouping is consistent, eliminating any flyers.

Step 7: Identify where your bullets struck the target by looking through a spotting- or riflescope, or physically examining the target.  In most cases than not, hopefully they land where you intended.

Step 8: If necessary, you can make corrections to further refine your zero.  To correct to the dead center of the target, adjust the elevation and windage knobs accordingly. 

  • If the bullet struck low and right, for example, you’ll want to adjust the elevation knob on top, upping the appropriate distance, so the elevation is correct.  Sometimes this is a guessing game.  The same holds true for windage.  Look on the knob for the indicator of which direction controls movement to the left/right on the side.  Since the bullet struck too far to the right, in this case, you’ll be adjusting the knob (left) to the appropriate distance.
  • Repeat Step 6 

Step 9: More often than not, you will need to make several refinements in order to find the perfect zero.

  • We recommend shooting three shots each time, again, to eliminate flyers.

 Note: Based on a hundred yard zero for…

  • Hunting rifles, a 1 ½ – 2 inch zero is a respectable group for hunting ammo
  • Long range hunting rifles, a 1 inch zero is a respectable group
  • Target Rifles, a sub-half minute is a respectable group

Step 10: Double check the tension on the screws to make sure they are tight.  If not, tighten and shoot again to make sure you have the same zero.  Make any adjustments, if necessary, and shoot again. 

BAM! You should have hit the bull’s-eye! You are zeroed in and ready to rock!

SUGGESTION: Before leaving the range, check that screws on your scope mount and your pillar bolts are tight. Repeat Step 6 to assure you have the perfect zero.

Why can’t I maintain that perfect zero?

A question we receive more than often is, “How do I maintain that perfect zero once I’ve already done it?  I seem to keep losing it.”

Keeping a perfect zero is tough, because there are a combination of mechanical and physiological factors that can alter the configuration of the relationship with the scope and gun.

  • Here is a video of the president of Horus Vision with a simple guide to zeroing your riflescope.

What changes your zero?


When you clean your gun, you are moving parts around, which most likely will shift your alignment


  • Any time you tighten:
  • Screws
  • Rings
  • Pillar Bolts

You are again shifting the original alignment, especially if you do not apply equal distribution of torque on component parts

To avoid unequal distribution, a torque tool is a good way to tighten everything up equally.

Mixing Ammo

To maintain the perfect zero, you cannot switch up the ammunition.  You have to shoot with the same brand, types, and weights.


Dramatic ranges of temperature will change your zero, because the air density affects the velocity of the bullet.  With increased temperature, there is a higher velocity.

Personal Shooting Practices

If you change up any of the following, you will most likely alter your zero:

  • Having another individual shoot the gun for you
  • Holding loose vs. tight on the shoulder
  • Holding loose vs. tight on the cheek weld

Differing your Gun Rest

Be sure to use the same kind of gun rest, whether it is a sand bag, mechanical rest, box, etc.  It should be stable and easy to repeat the same position every time.

Transportation of Weapon

Even if you don’t mishandle your weapon in the slightest, it is possible that a shift of zero could occur.

Turret Knobs Not Covered

If the turret knobs are not protected, they can rub against clothing, gear, storage bags, scabbard, etc.  If the knobs turn, it will definitely shift over your alignment and cause your point of impact to be off.

Defective scope

Having a defective scope is a possibility, but it is not likely.  If you have taken conscientious efforts to avoid any of the above in your shooting endeavors and you still aren’t accomplishing that zero, then maybe you really do have a defective scope and need to get that checked!



William C. Davis: A Tribute

On March 4, 2010, the renowned ballistician, engineer, and accomplished shooter, William C. Davis died at the age of 88.

Davis graduated from St. Bonaventure University in New York in 1941 with a degree in physics and mathematics.  He joined the Army 1n 1942 where he fought in WWII and left his military service after the war in 1946 as a Captain.

Davis went back to St. Bonaventure to teach until 1951, in which he worked as an ordnance engineer for the U.S. government for 23 years.  Some notable accomplishments during this time included heavily influencing the improvement of the M-16 rifle and standardization of small arms and ammunition between NATO countries.

In 1972, after many commendations and accomplishments, he retired from his work with the government and became self-employed, consulting and writing, among other tasks in the subjects of firearms, ammunition, and ballistics.  He eventually founded Tioga Engineering in 1980, operated by him and his business partner, Charlie Fagg.

Amidst his work within Tioga Engineering, he wrote for American Rifle Magazine, where he became the Contributing Editor in 1974, and went on to write over 50 articles from there.  In 1986, he earned the title of Ballistics Editor.

Other Noteworthy endeavors from Davis were the development of the Very Low Drag (VLD) bullet, which lead to a win for the U.S. International Shooting Team in the 300-meter competition and writing a majority of the NRA book, Handloading in 1981.  He also contributed the “Ammunition” section of the 15th Edition of Encyclopedia Britannica. 

Davis is also well-known for writing the first ballistic program for PCs, written in BASIC, which he offered for free to NRA members.  He went on to write a total of 14 ballistic software programs.  Horus Vision’s ATrag program actually derived from Davis’ algorithms and ballistic studies.  You can read A Brief Course in Ballistics for insight on his findings. 

On top of his technical expertise, he was also an avid shooter as a military expert in rifle, pistol, and carbine, a Lifetime Master in the NRA Smallbore Rifle and rated as a Class AA in NRA Hunter Pistol Silhouette.

This account of William C. Davis is an understatement of his impact, not only through his insight and accomplishments, but also through his character, as he was described to be respected because of his dedicated, patient, and considerate nature.

To read more about William C. Davis, click here.

To read about ballistics from the master himself, click here.

An Alaska Master Guide’s Experiences With Horus

As a Master Guide in Alaska and a Professional Hunter in Africa, I have watched both novice and very experienced hunters encounter the same problems. When pursuing long range shots, made under varying field conditions, most hunters cannot reliably make clean kills past 300 yards.  While using the Horus Vision System, I found with a little practice, the average hunter can double the effective range to make clean kills.

I must admit, the first time I looked through a Horus scope, I was skeptical and thought the reticle was too busy. A friend talked me into trying his new Horus scope. I was about to dismiss it as another gimmick, but the friend convinced me, if I tried it, I would like it.  I learned to shoot with open sights and did not like scopes the first time I tried them, but I decided to give the Horus Vision Scope a try. After a day of shooting the Horus scope at the range, I started to think it had real possibilities.  The more I used the Horus scope, the more I liked it, and am now sold on it. 

After testing a Horus Vision scope in the field for three years, it is my professional opinion that they are the absolute best scopes for big game hunting. Horus Vision Scopes are better than any other scope system I have ever used and are perfect for hunting under the tough and diverse field conditions of Alaska.  

While in the field hunting, fingers are cold, you are out of breath from a hard stock, or you cannot take your eyes off the game without losing it. Twisting knobs and counting clicks does not work for me… not a single client I have guided could reliably work knobs under Alaskan field conditions; Yet, under the same harsh real life field conditions, the Horus Vision system has worked time and time again for both me and my clients.

This is a list from this hunting season of 5 big game animals I personally watched harvested using a Horus scope:

  • 580 yards one shot kill
  • 337 yards one shot kill- Shot by 10 year old boy
  • 65 yards one shot kill- Shot by 14 year old girl
  • 96 yard shot
  • 890 yards one shot kill

The 890 yard shot was impressive. That shot was at a Caribou, wounded by a hunter from another party.  After trying for ½ a day to get closer to the wounded animal, we determined we could not get any closer in the open country where the wounded caribou was.  As the hunter got into a prone shooting position using a bipod, I used a pair of Leica range finding binoculars, mounted on a tripod, and determined the range to be 890 yards with no wind. The conditions were perfect for a long shot.  Knowing the exact range and using a Horus range card, the hunter knew what grid line to hold on the scope. The shooter did his job and 10 seconds after the shot was fired, the Caribou fell over dead.

Even the shortest shot of the season impressed me.  A few seconds before the shot was taken, the shooter was about to make a different shot at 380 yards. The range of the animal and the grid line to hold in the scope had been determined.  Just as she was going to fire, her dad saw a better trophy step out of the willows 65 yards from us. If we had used a scope that had the knobs dialed in for the 380 yard range, we would have had to readjust the knobs by counting clicks. The trophy at 65 yards had seen us at the same time we saw it. The shooter only had a few seconds to take aim at the new target and make the shot. With the Horus scope, all she had to do was hold on the correct grid line for the new range and make the shot. Being able to make instant adjustment with the Horus scope let her take the better trophy that day and that is just one more occasion where the Horus scope has impressed me.

My opinion, based on three years of use in the field, is that the Horus scope is the most practical hunting scope ever made, and is now the only scope I recommend to my clients and friends.

Jerry Jacques, Alaska Master Guide #110

Jerry Jacques’ 10-year-old son, Caleb, with his first Caribou taken in Brooks Range with a FNSPR .308 rifle with 26″ Lilja barrel, Manners stock, CDI bottom metal, and Horus Scope.

When You Do Everything Right and it Still Goes Wrong

It was one of those days when your pager or Nextel goes off and you know immediately that it is a call-out. It was in the early morning of August 4th 2008, 0600, when my Nextel starts talking. I hear my SWAT commander going through roll call to see if we are all “up” on our communications. I jumped out of bed and got dressed, grabbing my gear along the way. Luckily my department allows the SWAT team take-home cars and the bulk of equipment was already secured in the trunk. Along the way, I communicated with my sniper team leader, Sgt. Jeff Stanfield, and he advised me to meet him at Handy School along with the other snipers. During the night, patrol had located a suspect who held felony warrants for attempted murder. Patrol thought they had the suspect present at a housing complex. Handy School was located approximately three blocks away from our suspect location and we, the snipers, were meeting here, gearing up and moving into position for intelligence and cover until the entry team arrived. On this day I was using my Remington 700 custom by Engel Ballistics. It had a suppressor attached and was housed in an Accuracy International chassis. Sgt. Stanfield and myself teamed up and took a position 35 yards off the “1” side of the apartment complex. Due to the terrain and the buildings, all we could do was to get on a corner of another building in a prone position and use it for cover. Sgt. Stanfield took first watch while I began to set up to relieve him. During this month it had been extremely hot and humid, seeing that we had no shade and it was already 88 degrees at 0630 I began to communicate to Sgt. Stanfield about rotation of the watch. He agreed the rotation needed to be about every 15 minutes. On side one of the building, the suspect’s girlfriend had come to the door talking to patrol officers. They were telling her to come outside to talk and bring her children. By this time I had moved into position next to Sgt. Stanfield, my rifle parallel to his and was up on my elbows. I bolted a round into the chamber and “BAM!” the rifle fired.

At first I couldn’t believe it. I visually looked down at my trigger hand to confirm what I already knew. My finger was NOT on the trigger. Luckily, Sgt. Stanfield had been looking right at me and saw the entire process. He too saw that my finger was not on the trigger. The next thing that happened, about .0003 seconds after looking at my hand was to look up and see where my round had struck. I saw the girlfriend, I saw the two officers, everyone was ok, no one was screaming “I’m shot”, but where was the bullet? When the weapon fired, the police officers down range from us only heard the sonic crack of the bullet. Both officers told me later that they thought the suspect was firing on them with a .22 caliber pistol or rifle. Neither of the officers knew Sgt. Stanfield or myself were there and set up. One sergeant on shift called a “shots fired” over the radio but Sgt. Stanfield immediately called out a “weapons malfunction”.

Now you have to realize, the above paragraph took literally seconds to happen but it seemed as if to take forever. Also there were things that I don’t remember doing that Sgt. Stanfield tells me I did. For example, after the rifle went off and I checked down range, I don’t remember unloading the rifle and extracting the magazine. I later found the spent brass in my vest pocket, don’t remember putting it there. I moved the rifle out of the way against the building and took my place next to Sgt. Stanfield with a pair of binoculars. I don’t remember getting the binoculars out of my bag. In fact, all that was playing over and over in my head was the few milliseconds it took for me to look down at my hand and up where my barrel was pointing. I felt as if all the blood had drained from my head and I was sweating profusely; not from the heat but from the shock of what had just happened.

Sgt. Stanfield was a trooper, he stayed on the rifle the entire time we were in that location. He realized that I was mentally unable to get back behind a rifle. He told me later that he could “see it in my eyes” that I wasn’t really there. He located where my round had struck, into the trunk of a 1990 Dodge Intrepid. We would later discover that the round, a .168 grain Hornady TAP, remained in the trunk.

An extremely difficult thing to go through was the time. You see, I was next to the one person in the entire world who actually knew what had happened. I wasn’t worried what he thought. We laid there for three hours in the sun, by ourselves. All three chiefs were present and had heard what had happened from the command post. One deputy chief came over and asked if I was ok then asked what had happened. Another chief came over and just looked at my rifle not saying anything. Everything was going through my head at this time. The team wouldn’t trust me any more, hell I would be kicked off the team, oh hey! I could lose my job! If I didn’t lose my job, no one at the department would have any faith in my abilities ever again and on and on and on. Every time I looked up and saw officers talking, I believed they were talking about me and what had happened.

We discovered the building next to us had been evacuated so we decided to take refuge in it. We were out of the sun and out of sight from everyone. I took my rifle back to my patrol car and secured it in the trunk and grabbed my back-up rifle, a Sig-Blaser. After both of us had set up our sniper hide in the upstairs bedroom, Sgt. Stanfield called in to tell the commander that we were up and running and I had another rifle. The commander came back over the Nextel and asked if it was “one that wouldn’t shoot him” laughing about it. After hearing that, I felt some relief for the first time since this whole thing had happened. If he was laughing and joking with me, I felt it couldn’t be as serious as I was making it out to be. But it was, I just didn’t realize it yet.

After hours of standing watch over the apartment, the entry team shot tear gas inside and waited. After waiting the appropriate time, they made a dynamic entry and cleared the apartment. No one was home. The damage the entry team did was tremendous compared to my tiny hole in the trunk of the car. Still, my mind was in turmoil going through every step I knew to do and what I did to make this happen. The team rallied up and met back at the police department for a debrief. My shot was not mentioned. After the debrief, I was told by the commander that I would have to go through a shooting review board but “not to worry, its just a formality.” Sgt. Stanfield and I then did a detailed incident/offense report of the incident.

Right after the report writing, I called my wife and told her what had happened. At the time she was working out of town and I would not be able to see her or talk face to face for about three more days. She assured me that everything would be fine and for me not to worry even though she knew I would. I then called Patriot Arms and discussed my rifle. They told me to ship it to them and they would go through it inch by inch to find the problem. Later that day I had the rifle shipped second-day air in hopes it would get to them by the end of the week.

The rest of the week went by like a blur. Patriot Arms called me two days later and told me that they had got the rifle to slam fire two times with them and were now in the process of going over it. They felt sure that the flaw was in the trigger but were leaving nothing to chance. After the checkout, they did confirm the trigger being the problem. The trigger was a Jewell, and Patriot Arms did not “dig” into it. Instead they installed one of their field triggers and shipped me back the rifle along with the Jewell wrapped in an envelope. I opened the package and checked the rifle. The envelope was placed in my workroom.

All my focus now was on the shooting review board. Even though I had not injured or killed anyone, the department wanted this to discuss the issues related to the weapon malfunction. Sgt. Stanfield and myself had to “testify” so to speak about the incident in front of a panel of officers, supervisors, and a secretary. The questions were very basic as if they knew nothing about the workings of a bolt rifle or our sniper policy. In the end, I was cleared of any negligence or wrong-doing in my actions. This is where my nightmare started.

After the shooting review board cleared me, my mind just wrapped up everything very neatly, boxed it away, and stuck it in the deep corner of my mind. It was as if I told myself, “Phew, that’s over with, now get back to work and forget about it!” The problem was that I couldn’t forget about it. I found myself constantly thinking about the incident and replaying it in my head at night. I would lay down to go to sleep and suddenly jerk awake sweating and reliving the few milliseconds it took for me to look up from my rifle after the shot went off. I relived this over and over, the slow motion gaze, every detail of my barrel pointed down range at the front door. Then the “what ifs” started to play into the game. What if I had shot that woman at the front door? What if I had shot one of the officers standing close to her? What if the bullet had passed through the trunk and a window striking a child on the other side? What if, what if, what if? For weeks this went on and for weeks I slowly began to change. I felt guilty for feeling this way. I had not shot anyone! I had not hurt anyone! I had no right to feel this way when I have buddies that have actually been overseas and shot and killed the enemy! I had no right to feel this way when I have buddies that have shot and killed the enemy here doing their job! Why was I going through this tremendous guilt and pressure?  

It was about six weeks after the incident when I was talking to my wife over the phone. She was at work, out of town, and had asked me something when I snapped back at her. She asked me what was wrong and I told her nothing. She then told me something that struck home. She told me that I was not her husband lately. She told me that I was always edgy, quick tempered and restless. I asked her how long had I been this way and she told me, “Ever since your rifle went off on that call-out.” I immediately started crying. I don’t know how to explain it but the feeling was over-whelming. My wife told me to talk to somebody, anybody who I trusted, who would be neutral and could get me through whatever this was.  

The next day I talked to my Chaplin on my shift. He explained to me that what I was going through was normal all things considered. He explained that my worry and guilt was not coming from what happened but rather from what COULD have happened. I also called and talked to Derrick Bartlett, head of the American Sniper Association (ASA) and Snipercraft. Derrick explained to me that I was indeed going through post traumatic stress and exhibited all the signs of such. After talking and healing, I finally sent the Jewel trigger back to the company. I don’t know why I hadn’t already. I guess I was fearful that they wouldn’t find anything wrong with it. Along with the trigger I also enclosed a copy of the incident report and detailed letter of what had happened. A few weeks later I received a phone call from Jewell. The way it was explained to me was that the trigger had lost some of its spring tension as if some pressure was being exhibited on it. He asked me what ammo I was using and if I was shooting reloads. I told him the only ammo that went down the barrel was factory loaded. He couldn’t explain why the trigger had jumped from three and a half pounds to under one pound pull. He did say it was a little dirty but what was puzzling was the spring wear. I then told him that I had a suppressor on my rifle. Both of our light bulbs went off after this statement. We came to the conclusion that due to the back pressure coming in after the round is fired when using a suppressor could cause the spring tension to weaken.

A note to all those who use suppressors with Jewell triggers. Don’t learn the hard way, clean the trigger often.  

After the conversation with Jewell I felt as if I could close the book on my incident but after talking with Derrick I wanted to share my story with my fellow sniper brothers and sisters. I am one to meticulously document each and every shot that goes down the barrel of my rifle. I keep an accurate shot log and data book along with regular maintenance and cleaning. I am, so to speak, very OCD about my rifles and training. Two weeks prior to this incident, using the ASA course, I qualified with this same rifle without any malfunctions. I am not sharing my story because it is one of excitement or heroism- I am sharing my story because I want you to know that even when you do everything “by the book” as meticulously as I do, something can still go wrong. If I can give you one piece of advice it would be document, document, document – EVERYTHING! Stay safe.


Article written by Hal Howard

Hal Howard is a Horus Vision sponsored shooter, who recently placed in the Top 10 at SniperCraft’s SniperWeek held in St. Petersburg, FL from April 7-10, 2010.  We thank you for your hard work and wish you luck for the future!

Allegheny Sniper Challenger Takes First Place Using The Horus System

Jered Joplin, President of American Precision Arms, took first and second place in the 2010 Allegheny Sniper Challenge and he used the Horus System!

Here is his personal account of the event:

The Allegheny Sniper Challenge is a 2-day event held in the mountains of West Virginia.  The primary focus of the match is milling and high-angle shooting.  There are multiple unsupported stages, as well, but almost the entire course is unknown distance targets.  Targets range from 25 yards to 1250 yards.  One morning you actually start out on top of the Eastern Continental Divide. 

I took first place in 4 of the last 5 matches we attended there.  This year we took first and second place. 

The Horus system is tailor made for ASC.  It has been a major contributing factor to our success at this match and many others.  To be frank I would hate to shoot without it.  Thanks for making a wonderful product!

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Support Our Troops…One Soldier at a Time

If you’re looking for a good way to help out the troops, you can do your part by adopting a soldier and simply writing to them!  Letters, care packages, and any reminders of home help lift the spirits and boost morale to soldiers.

A good site that makes it easy to get paired up with a soldier and provides resourceful information on how you can make the experience worthwhile is Adopt a US Soldier.

You can also adopt a sniper at AmericanSnipers.org 

There are many other sites out there that provide information, forums, and resources on how to reach out to our troops and help make a difference in their lives.  Just type in ‘Adopt a Soldier’ in your favorite search engine and you can choose whichever organization suits you best. 

A Brief Course in Ballistics

As a long range rifleman, you should read this material carefully.  William Davis, a master of ballistics, explains in simple English a concise, informative explanation on how real time key factors influence bullet performance.  After reading this section, you will understand why Aiming Solution software, using the ATrag program is a must for extended range shooting.

Effect of Barometric Pressure on the Bullet’s Flight

Barometric pressure affects the down-range performance of a bullet because it affects the density of the air through which the bullet must travel.  The barometric pressure depends primarily on altitude and to a much smaller extent on the constantly occurring changes in the atmosphere that produces the barometric “highs” and “lows” that we hear about in weather reports.

The barometric pressures given in weather reports are not actual pressures; they have been adjusted to eliminate the effect of altitude.  The adjusted pressures given in weather reports everywhere are typically between 29 and 31 inches of mercury, although the actual barometric pressure in Denver (which is above 5000 feet) is typically about five inches less than the pressure in a coastal city such as New York.

Adjusted pressures are more suitable than actual pressures for weather forecasting, and users of barometers are generally advised to adjust their barometers to the pressure being reported in a local radio or television broadcast.  This adjustment makes their readings comparable to the readings of other barometers for weather forecasting, but the readings are then unsuitable for use in determining the atmospheric density.

The shooter who wishes to use a barometer for determining atmospheric density should take it to a facility where an accurate barometric reading is available, such as an airport control tower or a science laboratory at a college university.  He should explain clearly that the information he requests is the current actual barometric pressure and not the pressure after the adjustment for altitude has been applied.  He should adjust his instrument on the spot to the actual pressure, and henceforth the readings obtained from that instrument will reflect not only the effect of altitude, but also the effect of weather- related fluctuations in pressure, wherever the instrument may be.  It is this actual barometric pressure that is called for when the user elects to input the barometric pressure explicitly in the computer program that accompanies the HORUS sighting system.

Of course the user of the HORUS computer program may alternatively choose to input the altitude (as obtained from a topographic map, for example) rather than to input the barometric pressure explicitly.  In that case, the program will calculate the air density, with only slightly less accuracy, based on the inputs of altitude and temperature in which the barometric pressure is implicit.

Effect of Air Temperature on the Bullet’s Flight

The temperature of the air affects the aerodynamic drag (“air resistance”) encountered by the bullet in its flight.  There are two reasons for this.  The first and more important reason is that cold air is denser than warmer air, under conditions that are otherwise the same.  The second reason is that the speed of sound is lower in cold air than it is in warmer air.

The first reason is the more easily understood.  Most people probably know that almost every substance contracts and becomes denser as it is cooled, and therefore that cold air is denser than warmer air.  They also know that water is much denser than air, and almost everyone has noticed that much more effort is required to wade through deep water than would be required to walk along at the same speed surrounded only by the air.  The denser the medium through which a body passes, the more energy is expended (and the more velocity is lost) by the body as it passes through.

The second reason is not intuitively obvious, but it is a fact that cold air offers greater resistance to the passage of a bullet- especially a supersonic bullet- because the speed of sound is lower in cold air than it is in warmer air.  A full explanation of this phenomenon would be too lengthy to include here, but it is a fact that the force of aerodynamic drag on a fast-moving body depends upon the so-called “Mach ratio” which is the ratio of the speed of the moving body to the speed of sound.  Because the speed of sound is lower in cold air, the Mach ratio for a bullet at any particular velocity is correspondingly higher in cold air, and the force of aerodynamic drag is therefore greater.

Effect of Relative Humidity (RH) on the Bullet’s Flight

The relative humidity affects the down-range performance of a bullet because it affects the density of the air through which the bullet flies. Contrary to what many people suppose, humid air is less dense than dry air under the same conditions of temperature and barometric pressure, because the molecular weight of water is less than the molecular weights of the principal gases (nitrogen and oxygen) that comprise our atmosphere.

The effect of humidity on the down-range performance of bullets is small in comparison to other factors that affect the downrange performance.  The relative humidity has greater effect on air density at high temperature than at low temperature, but even at 90 degrees F.  The difference in density between completely dry air and completely saturated air is only about 1/10th of one percent.  For a typical .30-caliber 150-grain bullet having a ballistic coefficient of C1=.400 and a muzzle velocity of 2800 FPS, the difference in remaining velocity at 1000 yards is about 14 FPS, and the difference in drop is about six inches.

Effect of the Earth’s Rotation on the Bullet’s Flight

The Coriolis effect on the path of a projectile is a consequence of the rotation of the earth, and the fact that the surface of the earth is curved (spherical) rather than flat as we generally assume it to be for the solution of problems in exterior ballistics.  The magnitude and direction of the Coriolis effect depends on the location of the gun (its latitude) and the horizontal direction (azimuth) in which the gun is pointed.  The Coriolis effect is so small in comparison to other effects on the projectile’s path that it is not ordinarily considered except in the case of long-range artillery fire, but of course it does actually have some effect on all projectiles.

A somewhat simplified way to visualize the problem is to consider that, because of the earth’s rotation, a “stationary” target is not truly motionless as we normally assume it to be, but is constantly moving.  Consequently, the point on the target at which the projectile was directed when it left the muzzle will have moved some small distance (relative to the gun) during the time the projectile is in flight.  In this sense, the correction for the Coriolis effect is similar to the “lead” required to hit a moving target.

The Coriolis effect on projectiles fired either due north or due south is entirely horizontal, and the Coriolis effect (on a vertical target) on projectiles fired either due east or due west is entirely vertical.  The effect on projectiles fired in any other direction is both horizontal and vertical, the amount of each depending upon the horizontal direction (azimuth) in which the gun was pointed.

Effect of Wind on the Bullet’s Flight

The most important effect of wind on the bullet’s flight is to change its direction horizontally.  In the language of gunnery, angles in the horizontal plane are called angles of deflection, and so the effect of a cross-wind on the bullet’s path is correctly called wind deflection effect although the term “wind drift” is often used, rather loosely, instead.

To hit targets at long range, riflemen must learn to estimate the cross-component of the wind velocity.  The speed and direction of the wind can be measured by suitable instruments, or estimated by experienced observers from signs such as the motion of leaves and grass, and the appearance of “mirage” which is the wavy pattern of distortion that is seen through a powerful telescope, caused by refraction of light passing through waves of heated air as they rise from the ground.  Wind flags and other indicators are placed downrange during some types of rifle competition to aid in estimating wind effects.

Wind deflection depends upon the cross-component of the wind velocity.  A 10-MPH wind blowing from 3 o’clock or 9 o’clock has a cross-component of 10 mph.  10-MPH winds from 2, 4, 8 or 10 o’clock have a cross-component of about 8.7 MPH, while winds from 1, 5, 7 o’clock have a cross-component of 5.0 MPH.  Winds blowing from 6 or 12 o’clock have no cross-component.

Wind speed and direction are practically never uniform over the whole distance from the gun to the target, and so the rifleman estimating the allowance for wind must decide whether to concentrate his attention on the wind nearer the gun or on that nearer the target.  The answer is that the wind conditions near the gun have much greater effect than the conditions near the target.  Two hypothetical examples will illustrate this point.  For both examples, assume that the rifleman fires a 150-grain .30-caliber bullet having a ballistic coefficient of C1=.44 at a muzzle velocity of 2800 FPS, toward a target 500 yards away.

For the first example, suppose that a perfectly uniform 10-MPH wind blows from 9 o’clock across the first 100 yards of range, and that there is no wind whatsoever over the remainder of the range from 100 yards to 500 yards.  Consider now the situation when the bullet has reached 100 yards.  We see that the wind deflection is about 0.8 inch, which means that the bullet is now 0.8 inch to the right of the line from muzzle to target, but also that its path is curved toward the right at an angle of about 1.6 MOA.  With no further influence from the wind over the remaining 400-yard distance, the bullet’s path in the horizontal plane will be straight, but the horizontal angle of 1.6 MOA that it had already acquired at 100 yards will carry it 6.4 inches farther to the right of the gun-target line, for a total wind-defection effect of 7.2 inches at 500 yards.

For the second example, suppose that conditions from the muzzle to 400 yards are perfectly calm, but the 10-MPH wind from 9 o’clock blows across the range between 400 and 500 yards.  The horizontal direction of the bullet remains directly toward the target in the calm air out to 400 yards, where its remaining velocity is about 1959 fps, when it suddenly encounters the 10-MPH crosswind.  The bullet’s flight between 400 and 500 yards will be the same as that of a bullet of the same kind fired toward a 100-yard target at a muzzle velocity of 1959 FPS, for which we find that the wind deflection would be about 1.3 inches.

Effects of Shooting Uphill or Downhill

The vertical drop of a bullet below its line of departure is practically the same whether the target is uphill, downhill or at the same level as the gun.  That does not imply, however, that the sight adjustment or the allowance in aiming required to hit a target at any range is unaffected by the slope of the gun-target line.  The reason for this apparent contradiction is that the effects of an aiming allowance or an elevation adjustment of the sight are in a plane perpendicular to the line of sight, which, in the case of uphill or downhill firing, is not the same as the vertical plane in which the bullet drop is measured.  The reason we must take account of the slope of the gun-target line is illustrated in the following examples.

Suppose we are firing a .30-caliber 180-grain bullet having a ballistic coefficient of C1=.450 and a muzzle velocity of 2600 FPS, under standard sea-level atmospheric conditions, and that we have sighted-in the rifle at 200 yards.  Suppose further that we wish to shoot at a bull’s-eye on a tall vertical target 700 yards away on the same level as the gun.  We can calculate that, if we were to fire with the sight adjusted for the sight-in range of 200 yards, the bullet would strike about 147 inches low on the vertical target.  Therefore, to hit the bull’s-eye we must either (1) aim 147 inches high or (2) make an elevation adjustment of about 21 MOA (147/7) on our sight.

Now suppose that all the conditions are the same as those described above except that the tall vertical target is on higher ground at an uphill angle of 30 degrees.  Since the vertical drop of the bullet is the same as before, the bullet would strike the vertical target at a point 147 inches below the bull’s-eye if we fired with the sight adjusted for the sight-in range.  However, as we look upward at a 30-degree angle toward the target, the vertical line between the bullet hole and the center of the bull’s-eye would appear to be less than 147 inches long because of the angle from which we are viewing it.  We can calculate by trigonometry that a vertical line 147 inches long would appear to be only about 127 inches (147*cos 30 degrees) when viewed from a location 30 degrees below.  Therefore, we could hit the bull’s-eye by (1) aiming 127 inches high or (2) by making an elevation adjustment of about 18 MOA (127/7) on our sight.

By reasoning similarly, we can see that a 147-inch vertical line would appear to be about 127 inches long when viewed from 30 degrees above the target as well as from 30 degrees below, and therefore the same allowance in elevation must be made in either case.

Effect of Ammunition Temperature on Muzzle Velocity

As almost everyone knows, the muzzle velocity of a bullet is a factor of fundamental importance in determining the bullet’s path.  Therefore, the rifleman must have some knowledge of the expected muzzle velocity in order to decide where to aim or how to adjust his sight.  The best estimate of the expected muzzle velocity is obtained from carefully conducted velocity testing of the ammunition to be used in the field, fired from the rifle to be used in the field, and preferable with the ammunition at approximately the same temperature that it will be in the field.

There are two kinds of factors, random and systematic, that determine the muzzle velocity of any particular shot.  Some degree of random shot-to-shot variation in muzzle velocity is inevitable, and its affect on any particular shot is inherently unpredictable.  The rifleman can minimize the random variation by choosing ammunition that has demonstrated consistently good uniformity in carefully controlled velocity testing.

The ammunition temperature is the principal source of systematic variation in the muzzle velocity, assuming that the ammunition is being fired in the same rifle with which the basic muzzle velocity was established.  Unfortunately, the effect of temperature on the muzzle velocity varies widely from one load to another.  The rifleman minimizes the systematic effect of ammunition temperature on muzzle velocity by measuring the muzzle velocity with the ammunition at approximately the same temperature that it will be in the field.

A Memorandum Report written by Barbara Wagoner of the U.S. Army Ballistic Research Laboratory contains an analysis of a great many velocity tests of ammunition ranging in caliber from 5.56mm to 30mm, loaded with both single-base and double-base powders, and fired at various temperatures from -65 degrees F to +165 degrees F.  The effect of temperature varied quite widely from one load to another, as was undoubtedly expected from previous experience.  If all the various ammunition types are lumped together, however, the data indicate a typical velocity change of approximately 0.4 percent for a change of 10 degrees F in ammunition temperature.  This implies, for example, that a load which produces an average muzzle velocity of 3000 FPS at +70 degrees F would be expected to average approximately 3024 FPS at +90 degrees F and approximately 2976 FPS at +40 degrees F.  These differences are significant if targets are to be engaged at very long range, and that fact underscores the desirability of having reliable muzzle-velocity data for the ammunition at a temperature reasonably close to the temperature at which it will be in the field.

Effects of Ballistic Coefficient on the Bullet’s Flight

The ballistic coefficient of a bullet is the measure of its ability to move through the air with minimum resistance.  This resistance is called aerodynamic drag, and it’s most significant effect is to reduce the velocity of the bullet en route to the target and thereby to increase the bullet’s time of flight.  An increase in time of flight increases the vertical drop of the bullet away from its original line of departure, and therefore it also increases the vertical aiming allowance or sight adjustment required to hit targets at different ranges.  Aerodynamic drag also reduces the striking velocity of the bullet at the target, and it may thereby reduce the bullet’s terminal effectiveness, depending on the nature of the target.

Another important result of aerodynamic drag is that it makes the bullet susceptible to wind deflection, which is the horizontal change in direction of the bullet’s path, caused by wind blowing across the gun-target line.  Contrary to what many people suppose, the effect of cross-wind on the bullet’s path does not depend primarily upon the bullet’s time of flight, but upon the length of time that the bullet is delayed en route to the target by aerodynamic drag.  Any increase in the ballistic coefficient of the bullet tends to reduce this delay time, and it may do so even though the gain in ballistic coefficient is achieved at the expense of a lower muzzle velocity and a longer time of flight.  The following example will illustrate this point.

Consider first a .308 Win load that consists of a 150-grain bullet having a ballistic coefficient of C1=.400, fired at a muzzle velocity of 2850 FPS.  We can calculate that its time of flight to 700 yards, for example, would be about 1.027 seconds, and that its wind deflection in a 10-MPH crosswind would be about 51 inches.

Now compare this .308 Win load to another one using a 180-grain bullet of similar shape, which would have a ballistic coefficient of about C1=.480.  The muzzle velocity attainable with the heavier bullet at comparable chamber pressures would be only about 2600 FPS, and the time of flight to 700 yards would be increased to 1.070 seconds.  Nevertheless, the wind deflection would actually be reduced by about ten percent, from 51 inches to 46 inches at 700 yards, owing to the higher ballistic coefficient of the 180-grain bullet.

The ballistic coefficients of commercial sporting bullets in the U.S.A. are almost invariably based on comparison with the “G1 Standard Projectile” which has a specified diameter and weight, and a particular shape.  A ballistic coefficient based on the G1 projectile shape is properly identified as “C1” to distinguish it from other possible ballistic coefficients such as “C5,” “C6,” “C7”, “C8,” etc. that were once widely used in military sources referring to projectiles of various different shapes.  References to “the” ballistic coefficient (or sometimes “the B.C.”) of a commercial sporting bullet in the U.S.A. should be taken to mean the C1 ballistic coefficient unless the source gives specific notice to the contrary.

Most manufacturers of commercial sporting rifle bullets will provide values of the (C1) ballistic coefficients of their bullets upon request, and several manufacturers include that information in the reloading handbooks that they publish.  One manufacturer, Sierra Bullets, lists several different C1 ballistic coefficients for each bullet, each ballistic coefficient being intended to apply to a different velocity range.  For Sierra bullets fired at a muzzle velocity of at least 2500 FPS, the ballistic coefficient listed by Sierra for a velocity of 2500 FPS will generally produce satisfactory results using the HORUS computer program, over the ranges of practical interest to the rifleman.


The actual lead required also depends upon some factors that are determined by the actions of the shooter in firing the shot, and therefore cannot be predicted.  These include the particular shooter’s human “reaction time” (time between the instant he decides to pull the trigger and the instant that the trigger is actually pulled), and also the “lock time” (time between sear release and firing-pin impact on the primer) of the particular rifle and the “action time” (time between firing-pin impact on the primer and exit of the bullet from the muzzle) of the load.  The errors introduced by these unpredictable variables are minimized if the shooter uses the “sustained lead” technique of shooting at moving targets, and maintains a consistent “follow through”; the errors are maximized if the shooter attempts to “spot shoot” the target by taking aim at a fixed point ahead of target and then pulling the trigger.

Effect of Drift on the Bullet’s Flight

Drift is one of the phenomena that contribute to the horizontal deviation of a spin-stabilized projectile from the vertical plane containing its line of departure.  Drift is an incidental consequence of gyroscopic precession, which itself is otherwise essential for the satisfactory performance of spin-stabilized projectiles.  It is precession that forces the axis of a spin-stabilized bullet or artillery projectile to change its direction constantly as the trajectory curves downward, so that the projectile flies point-forward with its axis nearly parallel to the direction in which it is moving.  The angle between the axis of a projectile and the direction in which it is moving is called yaw.

After a spin-stabilized projectile has recovered from the effects of certain disturbances that it encountered during launching, it settles into an attitude of relatively small yaw that is called the yaw of repose.  At the yaw of repose, the axis in inclined slightly upward and toward the right for projectiles fired from a barrel having right-hand twist of rifling or upward and toward the left for projectiles fired from a barrel having left-hand twist.

It is the horizontal component of the yaw of repose that causes drift.  For projectile fired from a barrel rifled with right-hand twist, the horizontal component of the yaw of repose is toward the right, which causes the air pressure on the left side of the projectile to be greater than the air pressure on the right side, thereby forcing the projectile to drift toward the right.  The horizontal component of the yaw of repose tends to increase as the trajectory curves more steeply downward, and therefore the drift increases ever more rapidly with increasing range.  The horizontal directions are reversed, of course, for projectiles fired from a barrel rifled with left-hand twist.

The calculation of drift is relatively complicated, and to calculate it very accurately requires detailed information about the projectile that is not available for small-arms projectiles except for a few that have military applications at very long range.  The drift calculations incorporated in the Horus reticle and the corresponding computer program are based on typical values for a class of bullets generally used for long-range rifle fire; specifically, long-pointed boattail bullets of low-drag configuration such as the U.S. military 173-grain 7.62 mm M118 Match and the 650-grain Caliber .50 M33 Ball.  Because the whole contribution of drift to the horizontal deviation of the trajectory is relatively small, the lack of detailed information specific to each of the many different bullets that might be used for long-range rifle fire will not detract seriously from the practical accuracy of the results.

The Elements of Dispersion

In reference to shots fired at a target, dispersion refers to the scattering of the shots around the center of impact.  Small dispersion is synonymous with what is commonly called good accuracy, and large dispersion is synonymous with what is commonly called poor accuracy.  The causes of dispersion are sometimes divided into two classes.  The first, which can be called, aiming error, refers to errors in the direction in which the gun is pointed when it is fired.  The second, which can be called ballistic dispersion, refers to deviations of the bullet from its intended path toward the target after it has left the muzzle.

In the most restricted sense, the term aiming error may refer to the degree of accuracy with which the shooter has aligned the sight on his chosen aiming point at the instant of firing.  In a more general sense, however, the aiming error includes also the shooter’s error in choosing the point at which to aim.  Thus, for example, if the range to the target is much greater than the range at which the rifle has been sighted-in, and the shooter fails to adjust his aiming point or his sight so as to make proper allowance for this difference in range, then the mistake in elevation angle so introduced becomes a part of his total aiming error irrespective of the steadiness with which he aims the rifle.

The ballistic dispersion depends primarily upon the quality of the rifle and the ammunition.  If the shooter can make satisfactorily small shot groups at short ranges such as 100 yards or 100 meters, he has established the quality of the rifle and most of the properties that determine the quality of the ammunition.  The accuracy of the rifle/ammunition system at long ranges cannot be inferred reliably from small groups fired at short ranges, however, because the vertical dispersion at long ranges depends very heavily upon the shot-to-shot variation of muzzle velocity, whereas the short-range accuracy is often quite insensitive to variations in muzzle velocity.

Whereas the ballistic dispersion depends upon the quality of the rifle and ammunition, the aiming error depends upon the skill of the shooter and the capabilities of the sighting system.  It is assumed that serious users of the HORUS sighting system will already have acquired the necessary skills in marksmanship.  The HORUS reticle, and the related computer program, are intended to reduce the total aiming error by helping the shooter to select the correct aiming point (or the correct sight adjustment) based on the conditions under which the shot is to be made.

The information about the prevailing conditions must be provided by the shooter or by his coach or observer.  The various elements of this information are more or less important, depending upon the range to the target and the relative magnitude of each element’s effect on the bullet’s path.  The muzzle velocity, the ballistic coefficient, the range, the wind conditions and the target speed (in case of a moving target) have relatively large effects on the bullet’s path relative to the gun-target line and therefore must be most accurately known.

Moderate differences in the uphill and downhill slope of the gun-target line have only moderate effects on the bullet’s path, and therefore reasonable estimates of the uphill/downhill angle will generally be satisfactory.  The drift (which is caused by the gyroscopic precession of the bullet) and the Coriolis effects (which is caused by the rotation and sphericity of the earth) have relatively trivial effects on the bullet’s path, and they are not often considered in calculation of the relatively flat trajectories that are characteristic of rifle fire.  Nevertheless, provision is made for taking account of all these factors in the HORUS system because all of them make at least some small contribution to the total aiming error which the system seeks to reduce insofar as is practicable.

It must be recognized, however, that neither the aiming error nor the ballistic dispersion can ever be reduced to zero, and therefore whether or not a target will be hit by any particular shot is a question of probabilities, depending on the size of the target, the proficiency of the rifleman, the accuracy of the information about the conditions under which the shot will be made, and the ballistic dispersion of the gun/ammunition system.  The rifleman should learn by experience and recognize realistically the outer limits of the ranges at which various types of targets can be engaged with a reasonable probability of success.

Weather Standards

A. The ATRAG program offers three choices for atmospheric conditions.  The first two choices define “Standard Atmosphere”.

A “Standard Atmosphere” defines the atmospheric conditions for which the “standard” firing tables and other tables giving “standard ballistics” are computed.  In the solution of actual gunnery problems, the corrections for the prevailing atmospheric conditions are made on the basis of the agreed standard atmosphere.

  1. The ARMY STANDARD METRO atmosphere was established at the U.S. Army Aberdeen Proving Ground and was used for many years by the U.S. Army as the atmosphere for which all standard firing tables were computed.  This standard atmosphere was also adopted by the manufacturers of commercial ammunition, and it is still in use by the major manufacturers of commercial ammunition and bullets.  IN the Army Standard Metro, the atmosphere at sea-level has a temperature of 59 degrees F., a barometric pressure of about 29.53 inches of mercury, and a relative humidity of 78 percent.  The atmospheric density under these conditions is about .0751 pounds per cubic foot.
  2.  The “ICAO STANDARD ATMOSPHERE” was defined by the International Civil Aviation Organization during the early 1950’s, and it was adopted in the late 1950’s as the standard atmosphere for all of the U.S. armed forces.  In the ICAO standard atmosphere, the atmosphere at sea-level has a temperature of 59 degrees F., a barometric pressure of about 29.92 inches of mercury, and a relative humidity of zero.  The atmospheric density under these conditions is about .0765 pounds per cubic foot. 

B.  The Horus Vision System offers the rifleman two choices to factor weather data.

  1. Altitude and temperature choice is recommended when computing a range table.  This table is usually attached to the stock of your rifle.

Real time barometric pressure, temperature, and relative humidity choice of field data to help calculate a very accurate firing solution.  Recommended when long range precision is a must.

Is My Variable Power Scope in the 1st or 2nd Plane?

First Focal Plane

If your variable power scopes is in the first focal plane (objective plane), all elevation and windage adjustment clicks are valid regardless of the power setting and point of impact will not change.

Note: Most European and a few American scopes are in the first plane.  A first plane scope can usually be identified by looking through the scope while changing the power.  If the reticle changes size, the scope is in the first plane (objective plane).

Second Focal Plane

If your variable power scope is in the second plane (ocular plane), the values of your elevation and windage adjustment are not valid for all powers of your variable scope.  When shooting at different powers, your point of impact will most likely change.  To find the exact power setting where the calibration values of the adjustment knobs are valid and true, you must read your scope instruction manual, call the manufacturer, check the catalog, etc.  The exact power setting is extremely important.

If your scope has Mil-DOTS, that exact power setting is extremely important when using Mil-DOTS to determine range.  A wrong power setting will yield an incorrect answer for range.

WARNING: When long range shots are to be taken after the proper number of clicks have been dialed in for elevation and windage, you must be sure your vari-power adjustment is set to the correct power.  Failure to use the correct power means your bullet could miss the target.