I recently made a trip to Camp Butner, the North Carolina National Guard Training Center, and spent a day on Range 4, the Known Distance 1,000-yard range. The objective of my trip was to get a set of zeroes. It occurred to me that those who don’t know about the technique of shooting a rifle or pistol at long ranges might understand the concept of a basic zero, but not understand the significance of a set of zeroes. Non-shooters think of a zero as nothing, but to a shooter, a good zero is everything and no shot can succeed without one. The shooting definition of a zero would be the position of the sights relative to the trajectory that allows the bullet to go where the sights are aligned at a specific range.
Basic hunting rifles and pistols have only rudimentary sights. Those sights can be filed and drifted to change their position and in the case of many defense pistols, they are adequate as they are. At 10 yards, a distance beyond the normal distance of the average defensive encounter, almost any pistol is capable of hitting an apple, and an apple is far larger than the vital area of an assailant. The simple sights that come on hunting rifles are rarely used, the vast majority of hunting is likely now done with optical sights. It wouldn’t make sense for rifle makers to spend a lot of money on sights that are almost never used.
Almost all pistols and rifles designed for competition have adjustable sights. Most of these sights can be manipulated in a documented method to allow adjusting them to compensate for the effects of both gravity and weather on where a bullet strikes and then return to the original zero for normal conditions. To do this, you need a starting point and that starting point is your base zero. The zero I sought was on a Bushnell 5-15 Elite Tactical scope mounted on a Rock River CMP Match Rifle. Since this was for NRA Any Sight/Tactical High Power Rifle competition, my base zero was to be 200 yards, the shortest distance normally shot in an NRA High Power match.
The process of getting the base zero is no different than any other sighting-in procedure. If I were a perfect shooter, I could fire one shot under ideal conditions, adjust to it, and use it as my zero. Instead, I need to fire a number of shots, documenting them and using that information to determine my base zero. Once I have this, I can loosen the micrometer-type turrets and adjust the scope turrets on the Elite Tactical scope and set them to zero on both elevation and windage. Once I’ve done this, I can adjust the windage and elevation knobs of the scope during the course of the match to compensate for conditions and always find my way back to my base zero at 200 yards.
When great precision is required, the level of refinement of the zero is much more demanding than a normal hunting zero. Long-range rifle and precision pistol shooters maintain a data book to document the history of their shooting and use this information to keep their base zero as well defined as possible. Ammunition, weather changes, and physical changes to the firearm can cause the zero to migrate in tiny amounts and to the precision shooter, a tiny amount is the difference between a hit and a miss. The zero is constantly monitored and fine adjustments are made based on the recorded information in the data book.
In the instance of my needs for high power competition, I also need to know how much to move the scope’s turrets to center the group at 300 yards and at 600 yards, the other two distances involved in a conventional high power rifle match. The farther a bullet goes, the further it falls. I know, based on tables provided by the ammunition company, that the bullet would fall about nine inches (or three minutes of angle) from 200 yards to 300 yards. I added three minutes of angle (12 clicks on a 1/4-minute value scope) and fired my first shot at 300 yards. It was about two inches low and I added another two clicks (or a half-minute) and my group was centered. I fired two 10-shot groups in the allotted time limit to confirm my findings.
The next yard line is at 600 yards. The prescribed additional elevation for the load and bullet I was firing was 60 inches (or 10 minutes) to put the bullet in the center at 600 yards. My first shot was low and I added another two minutes (or 12 inches). The next shot was high by about the same amount. I returned to the first setting and shot a well-centered shot. To be certain of my setting, I continued for 20 shots to make sure any errors were in my shooting and not in the data I would be using. In the process of this sustained firing, I determined that I needed another half-minute of elevation to best center my group. During this time, I was constantly evaluating the wind to determine the validity of my windage zero. The further you are from the target, the more critical your base windage zero is. This isn’t easy because it is a rare thing to fire a shot at 600 yards without the wind affecting where that shot’s going to land.
I now had my set of zeros for shooting 200, 300, and 600 yards. In total, it will take an elevation change of 81 inches (or 13.5 minutes of angle) to compensate for the distance from 200 yards to 600 yards. To be sure this information is current, I must maintain a data book and record every shot to make fine adjustments to my zero. Any change made at 200 yards must be compensated for at 300 and 600 yards, unless that change relates to a temporary condition such as weather or my shooting position. Changes of impact that are a result of external conditions like wind and temperature must be discounted from the equation.
These procedures are what long-range precision shooters have used for a hundred years. During the upcoming NRA High Power Championship, I’ll get two sighting shots at every stage, but those two shots are intended to help me deal with conditions specific to the time I’m shooting. Without a carefully recorded and documented zero, I would be at a tremendous disadvantage.