Something that has often puzzled me is the apparent lack of understanding among some members of the shooting/hunting fraternity of the proper way to sight-in a rifle. I’m not talking about the simple act of adjusting the sights or scope until you hit something at a convenient distance and being done with it. I’m talking about first understanding your rifle’s capabilities and limitations, then sighting-in your rifle to take full advantage of those capabilities, while minimizing the effects of its limitations. All you need is a little knowledge of ballistics, a couple of laws of physics, and a tad of plane geometry.
The study of firearm ballistics is divided into three main areas: interior (or internal), exterior (or external), and terminal. Interior ballistics deals with what happens inside the firearm, from the instant the primer is struck, until the bullet leaves the barrel. Terminal ballistics deals with the interaction of the bullet with the object it hits. This would involve such things as the study of wound data. Exterior ballistics deals with the projectile from the time it leaves the barrel until the moment it strikes something, and this is what we want to address here.
All moving objects are affected in the same way by the same two physical properties, namely gravity and air. As soon as a bullet leaves a gun barrel it begins to slow down at a predictable rate, based on two factors, the ballistic efficiency of the bullet (known as the ballistic coefficient) and the air density. The drier the air, the faster a bullet will lose its velocity, because dry air is heavier (denser) than humid air. This is of no consequence to the average shooter or hunter, but for someone trying to hit a target more than a few hundred yards away, it can become a consideration.
Gravity begins to pull the bullet downward the moment it leaves the barrel. The only way a shooter can hope to hit a target at any appreciable distance is to hold the barrel at an incline to the horizontal. The degree of inclination necessary is dictated by the bullet’s ballistic coefficient, its initial velocity, and the distance to the target. The amount of the inclination is controlled by adjusting the sights. Of course the sights have no interaction with the gun in any way, but they do require the shooter to point the rifle in a specific direction in order to line up the sights between the shooter’s eye and the target. The only rule to remember is that moving the front sight in any direction causes the point-on-impact (POI) to move in the opposite direction, while moving the rear sight in any direction causes the POI to move in the same direction. Sighting systems with internal adjustments, such as scopes and red dots, require the shooter to change the barrel alignment in the same way, they just do it internally.
Several years ago, I accompanied my cousin to a local shooting range. He had purchased a new scope for his rifle and wanted to sight it in for an upcoming bear hunt. The range had target butts at 25, 50, and 100 yard distances. At the time, we were the only shooters present, so the local range safety officer was hanging around to see if we needed any assistance. He was a nice young man, and I always appreciate such gestures. I suggested that we set up initially at the 25 yard range in order to make sure he would be “on the paper” as they say, when we moved to 100 yards. Our initial shots at 25 yards were several inches low, and the officer said, “You’ll be even lower at 100 yards.” I was quite surprised by his remark. He apparently thought that the bullet was on its way down at 25 yards. For that to be true the barrel would need to have zero, or nearly zero, inclination, a near impossibility with any conventional sighting arrangement. My cousin was shooting a 30-06 with a typical factory hunting load. We adjusted the windage and cranked up the elevation until we were just slightly low at 25 yards, then moved to the 100 yard range, knowing that we could expect the bullet to strike within a few inches of the point-of-aim (POA) at that range. Any shot he was likely to get while bear hunting would be less than 25 yards, so he didn’t think it was necessary to be precise. We shot at 100 yards, tweaked the windage adjustment, adjusted the elevation to hit a couple of inches high, and called it good.
There are three paths that are crucial to understanding exterior ballistics. They are the line-of-sight (LOS), the line-of-bore (LOB), and the bullet path, or trajectory. The LOS extends from the shooter’s eye to infinity. The LOB extends from the chamber (the area at the rear of the barrel that holds the cartridge at the time it’s fired) through the center of the bore (the hole that was drilled, or “bored”, through the center of the barrel) to infinity.
There are two more terms that must be understood in order to effectively sight-in a rifle. These are midrange trajectory (MRT) and point-blank range (PBR). The term “point-blank” is one of the most misused and misunderstood terms in common usage. We hear it used often in tv shows, movies, and newscasts. It’s used in place of “very close range”, and it’s so common that we’ve all come to understand its implied meaning. I’ve read several differing opinions as to its origin, from an artillery term to an archery term. But as it applies to the subject at-hand, it refers to the longest range at which an acceptable hit can be made without resorting to “holdover”, that is, intentionally aiming at a point above the intended target in order to hit it.
Your firearm will only be “right-on” with the sights at two distances, once when the bullet crosses the LOS on the way up, and again when it crosses it on the way down. Knowing your bullet’s trajectory can pay off in more ways than just the obvious. In 1969, I was on a deer hunting trip with my father-in-law and his father-in-law. We were at my father-in-law’s remote cabin in Michigan’s Upper Peninsula. It was the day before Thanksgiving, and we had no meat left. I was sneaking my way along a lake shoreline when I spotted a big snowshoe hare. All I had was my 30-06. I knew that there wouldn’t be enough meat left to bother with unless I shot it in the head. I handloaded all of my ammo even way back then, and I knew that my bullet would leave the muzzle 1.5 inches low, and first cross my LOS at about 30 yards. I did a little mental “guesstimation” for the holdover, and made a perfect head shot. We had fresh rabbit for Thanksgiving, with no wasted meat.
Take advantage of your rifle’s capability by maximizing your acceptable MRT. It makes no sense to zero a flat-shooting rifle at 100 yards. Earlier, I talked about helping my cousin sight-in his rifle for a bear hunt. I said we adjusted it to hit about 2 inches high at 100 yards and called it good. He was expecting to get less than a 25 yard shot at a bear. But I knew that a few months later he would be deer hunting in Northern Michigan with the same gun. He wasn’t likely to shoot it again between hunts, and he was unlikely to take a shot at more than 100 yards while deer hunting. Since he had a new scope on it, he was unlikely to ever shoot it again except at game, or unless something happened to make him think it lost its zero. I knew that with his load hitting two inches high at 100 yards, he would have an MRT of less than 3 inches, and his PBR would be about 250 yards, where he would only be about 3 inches low. My cousin now had his rifle set up to be able to take a shot out to 250 yards and hold dead-on.
The biggest advantage to maximizing your PBR is that you really don’t have to make an accurate estimation of the distance to your target. Many of us aren’t very good at estimating distance anyway, and in certain conditions, such as weather and terrain, it can be difficult for anyone.
By far, the longest shot I ever took at game was on a mule deer in 1976. We were packing up to leave and I had an empty tag. There was less than an hour left of shooting time, and I was glassing all around us. I spotted a herd in a huge hayfield so far away that I couldn’t see them without my binoculars. There was a series of coulees running between us, and I decided to try to get close enough for a shot. I didn’t have much time, so I hurried through the coulees until I came to the end. I popped my head out to see a sizable herd without a sign of antlers. I decided to fill my tag with a doe, but they were still too far away. I started crawling from one sagebrush to another, until I got to the last one between us. It was still a long way to the nearest large doe. I sat down behind the sagebrush and got into a hasty sling. I was shooting a 30-06 and my handload was stuffed with a Sierra 165 grain HPBT. I knew that I would be about 3 inches low at 250 yards, but I knew she was much further than that. I decided to set my horizontal crosshair on top of her back and hope for the best. My shot took her low in the lungs. Based on my hold and where it struck her, I think she was an honest 350 yards away. That’s a reach for an Eastern hunter like me. Had I not been familiar with my rifle’s trajectory, I probably wouldn’t have come home with any venison.
For the hunter expecting to take shots at long range, it’s a good idea to sight-in your rifle in atmospheric conditions reasonably close to what you expect them to be while hunting. Another consideration for long range shooting is the temperature of your ammunition. Pressure is what makes your rifle and ammunition work, and things that affect that pressure can affect its performance. Your ammunition loses pressure, and thus velocity, as its temperature decreases. I’m not talking about air temperature, but the actual temperature of your ammunition. Some of the newer powders on the market claim to be less temperature sensitive, but it’s still a good idea to be aware of this anomaly. Cartridges spending all day in a rifle magazine in zero weather will have less velocity than identical cartridges carried in a pocket close to your body. If you handload, a load that is near max in cold weather can be above max in hot weather. Anything that changes pressure also changes velocity and accuracy (uniformity) of your load.
Unless the bullet is traveling either straight up or straight down, its path is a continuous curve, from the end of the barrel until it stops. A bullet (or any other object with no means of propulsion) going through the air can never travel in a straight line (except when it’s going straight up or straight down). Gravity is constantly acting upon it in a straight downward direction.
I remember watching a very interesting interview a couple of years ago with one of our military’s most accomplished snipers. He had been deployed to Iraq, and he was being interviewed because of the many incredible shots he had made while in combat. He was talking about different circumstances that can complicate the shot. He stated that a sniper must remember to compensate on uphill and downhill shots, since a downhill shot will hit higher because of less drop, and an uphill shot will hit lower because of more drop. I was shocked when he said that. Our snipers go through some of the most intensive and thorough training of any members of our military. Beyond all doubt, I knew that he knew better. He wasn’t just drawing from experience during this interview, but also from theory, and he got mixed up. Perhaps he was a little nervous about being in front of the camera. The questions asked of him were also “on the fly”, so he had no foreknowledge of them to help him prepare. I also wonder if he hadn’t had many opportunities while in combat to take any steep uphill shots. I would think that if he had, he probably would have been far less likely to have gotten confused on that point. He knew better, and I’m sure he watched the interview later and winced a bit at that point. I don’t want to sound critical of him. He was a very knowledgeable, talented, and capable hero in every way.
But back to my point…both uphill and downhill shots are affected by gravity in the same way. Gravity pulls a horizontal shot at 90 degrees from the LOS because it’s pulling it straight down, or largely perpendicular to the LOS. A case can be made that it’s only pulling it exactly 90 degrees from the bullet path at the MRT point, but this is an extreme case of splitting the proverbial hair. The steeper the angle (either up or down) of the shot to the horizontal, the less the bullet drop from the LOS. The closer the angle comes to 90 degrees, the less the drop from the LOS becomes. When it finally reaches 90 degrees (straight up or down), the drop from the LOS becomes zero. At this point, the difference between the LOS and the LOB is simply the distance the sight is mounted above the bore. It will be constant because the 2 lines will be parallel.
You can observe this same gravitational effect by putting a high-pressure nozzle on your garden hose, adjusting it to a narrow stream, and varying its trajectory between horizontal and vertical. If you go all the way vertical, you’ll of course get wet! A bullet shot at a 45 degree angle upward will drop slightly more from the LOS than a bullet fired at a 45 degree angle downward because the greater velocity loss results in gravity having more time to act upon it, but this is splitting the proverbial hair once again. By far, the greatest factor to consider for either an uphill or downhill shot, is the decrease in drop from the LOS compared to a horizontal shot.
Within short distances, a bullet seems to follow a straight line, but that’s a physical impossibility. It’s always either rising or falling with respect to the horizontal and the LOS. This means that a bullet never rises above the line of bore, and it is only level with it as it leaves the muzzle. From that point on, it’s constantly falling further and further below the line of bore, until it stops. This means that if the bullet travels far enough, it will cross the line of sight twice, first on the way up, and again on the way down. Note that this is only true of conventional sighting systems mounted above, and in the same vertical plane, as the bore. Sighting systems that are offset from the vertical plane introduce a complication. The bullet can only cross the line of sight either once, or not at all. If it does cross it, it will converge toward it until it crosses it, then diverge from it until it stops. For this reason, volley sights that were mounted on some military rifles in World War 1 (the British SMLE comes to mind) were never meant to be “right on”. Their LOS and LOB were meant to be parallel. In this way, the built-in windage error of the sighting system would only be an inch or so for the entire path of the bullet. This is the reason why it had to incorporate both a front and rear sight. With this arrangement, the only difficulty is the range estimation.
If you have a backup or auxiliary sighting system mounted in a way that puts it on a different vertical plane than the bore and you expect to have it useful at both short and long range, this can be a problem. Since the bullet will only cross your LOS once, the bullet and your LOS gradually converge, cross (the zero point), then diverge. You should sight-in this system as best you can to have the same long range zero point. This puts the maximum divergence right at the end of the gun barrel until at some point beyond your PBR.
When using handguns for short range or defensive shooting, the bullet’s second crossing point, where it drops back down below the line of sight, is immaterial. The gun is actually “zeroed” at the point where the bullet crosses the LOS on the way up. Casual handgun shooters who don’t realize this are sometimes amazed when they attempt to hit a target at a much greater distance without adjusting their sights or engaging in holdover. When using typical combat handgun sights designed for close range shooting, the inclination imparted to the barrel is quite substantial in order to cause the bullet to cross the line of sight close to the gun. This will cause the point where the bullet crosses the line of sight again on its way down to be a considerable distance downrange. A 9mm Luger +P 100 grain bullet at an initial velocity of 1475 fps and crossing your LOS at 10 yards won’t cross it again until it’s about 55 to 65 yards away.