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How to Zero a Gun...
Zeroing relates to all three aspects of ballistics: internal, external, and terminal, check out wiki on each for more general detail. Although you can zero both iron sights and scopes, this article will concentrate on scopes.When you mount a scope on a gun, imagine a line extending straight forward to infinity from the scope. Now, imagine another line extending to infinity from the bore (inside of the barrel of the gun).
The scope line is obviously above the bore line. Ballistics show that each bullet travels in a parabolic arc of various shapes and slopes, "rising" above the line of sight, reaching a peak, then falling, all due to the effect of gravity on the projectile vs. inertia and weight. If your "site line" on the scope is pointed down a bit, the bullet obviously crosses that line in two places-- on its way up (called the near zero), and on its way down (the far zero). Contrary to popular belief, due to the laws of gravity, a bullet never really "rises" above the bore line; the "rise" is above the sight line of the scope.The exact definition of a zero is the point at which there is NO RISE and NO DROP. If you just put a scope on without adjustment, and it's perfectly straight, and is 3" above the bore, the bullet will hit 3 inches low close up, because the bore is that much lower than the scope. In reality, these lines are never parallel, the scope is always tilted down to intersect with the bullet curve and bore.
Most people think of zero as the point where your bullet will actually hit the target (noun) when you aim at it, which also might be true, if you "zero" (verb) the scope to that point! The old process of zeroing was to go out to the range, look at the target through the scope (or bore!), start firing, and adjust the scope up or down, left or right, until your shots hit the bullseye.
If your target was at 100 yards, your gun (actually your scope) was then "zeroed" for 100 yards. Since the bullet travels in an arc, remember that there was also a "near" zero, at about 25 yards for most bullet weights and ballistic coefficients (shape ratio of the bullet). Since your bullet always goes up, then down, the point where you "zero" your scope will create a division between this rise and fall.
For example, if you zero a 50 BMG M2 military rifle at 1,000 yards using a 709 grain bullet (M2 FMJ-BT 2850 FPS, source: ballistic software run on Quickload), the bullet will be 92.9 inches "high" (rise) at 500 yards, and have a "drop" of 191 inches at 1300 yards.
Zeroing a single shot Noreen 50BMG at 100 yards will give "drops" for everything beyond that range-- 46.68 inches at 500 yards, and 262 inches at 1,000 for a Hornady 750 grain AMAX, for example. "Drops" in ballistic tables are given in both MOA and inches, because scopes are calibrated in both. To hit a target before or after your zero, you'll need to either adjust your scope clicks for that rise or drop, or "hold over" (aim above or below your target) a certain number of "scope dots" to make up for the non-zero bullet rise or drop.
Another method of zeroing is to simply remove the bolt on your rifle, look through the barrel, and adjust the sight picture of your scope to coincide with the bullseye. Of course, your rifle will then be "zeroed" at the distance you set your target-- often 25 yards.The most modern zero techniques use bore sighters-- either lasers that protrude from the barrel, or optical collimators like the Bushnell or Leupold that magnetically attach to the crown of the barrel, creating a little "target" picture when you look through the scope.
These are guaranteed to "get you on the paper" (usually at 25 yards, seldom at 100) without firing a shot. In the latest version of the Leupold bore site, there is a "guaranteed one shot zero" because after you are "on the paper," you can actually look through the bore sighter, adjust the scope to superimpose the previous bullseye on where the shot actually hit, REMOVE the boresight, and shoot again.
Once your scope (gun) is zeroed at a certain yardage (typically 100 for most hunting guns, 200 to 500 for large sniper rifles like the 50 BMG, 25 for pellet guns), if you go hunting or target shooting, you will need to estimate the range to your target, then adjust your scope with windage and elevation clicks to compensate for windage and elevation differences compared to your zero. Ballistic charts tell you the rise or drop based on your bullet factors, and calculators like the milldot master can convert between distance, MOA, mils and bullet drop in seconds.
The world's best shooting competitors even use handheld computers with ballistic software to exactly compensate for everything affecting zero, from wind and humidity, to the rotation of the earth.
Q. Since most expensive and long range scopes are in Mil Dots, why are the adjustments in MOA, and how do they relate?
A. One strange answer is that DOD refuses to issue sniper teams range finders (even though the Bushnell 1500 is about 400 bucks!), so mil dots are still being used to estimate yardage. Since a mil target size is 36 inches high (3 feet), (as in distance between the two dots), at 1,000 yards, if a 6 foot tall man covers 2 mils in the scope, you know he's standing at 1,000 yards. Some folks ask "at what focus?" (there IS such a thing as a dumb question)-- it doesn't matter, because the reticle has the same mil readings regardless of setting. However, you should set parallax to correct yardage before zeroing, zero the first time mid-range (if 20-40X, zero at 30), and for final, at max.
MOA clicks are totally dependent on your bullet. Adjusting your scope for bullet drop from it's current zero is done with MOA clicks, even with a mil dot reticle. If you know your target is at 700 yards (from your mil dots, or range finder), you have to "bring up" (or come up) on your target by the number of inches that the bullet falls at that distance. For convenience, let's say your bullet drops 210 inches at 700 yards. This means, from your scope zero (of 100 yards in this case), it has dropped 210/7 (700 yards, at one inch per MOA per 100, is 7), or 30 MOA. If you are going all the way from 100 to 700 on your scope, you've got to "come up" (as in, "bring the bullet path up" to your scope's line of sight/crosshairs) by 30 MOA. with 1/4 clicks, that's 120 clicks, with 1/8, it's 240.
Where do mils come in? Well, if you'd rather "hold over" your rifle with MILS instead of adjust your scope with MOA, you can remember that 1 mil = 3.438 MOA, and if your MOA come up is, say 12 MOA, the mil come up would be 12/3.438, or about 3.4 or 3.5 mils-- the number of mil dots (unless your scope has MOA dots!) you have to hold over to compensate for the drop. We chose the 12/3.4/3.4 example because it's easy to remember. Now you know both, and can handle ANY type of scope. For small targets, another "closer" 12/3.4 relationship is that at 100 yards, a 12 inch target covers about 3.4 mils of reticle in your scope. At that yardage, a 12 inch bullet drop also is about 12 MOA (actually 11.5, but close!).
If you read this stuff and ask "but doesn't that depend on the bullet?" -- you're getting to an important advanced topic-- which factors are constant and which are variable, and which are dependent and independent variables. This confuses a lot of shooters! The two variables that "move together" are distance and bullet drop, and they are related to the constant (math multiplication or division) relationships between MOA and mils (1/3.4 ratio). Playing with the Mildot Master gives you a GREAT visual of how these variables interact. Once you know drop, you've already factored type of bullet into the equation, as the BC is implicit in the drop number, as well as the weight (bigger grains drop faster).
Q. What is a "Flat Shooter?"
A. Most people talk about the 460 Smith Magnum or 375 H and H as "flat shooters." What do they mean by this? Simply: a 100 yard difference in a light .223 can mean a two foot difference in rise or drop, meaning the ballistic curve is very "curved." Flatter means less rise and fall across the whole projectile path. For example, a 375 H and H zeroed at 300 yards will only be about 5 inches high at 200 yards, a pretty FLAT trajectory indeed! You might argue that this is wrong, because we didn't specify bullet weight. But we tricked you! The old (1912) 375 is a mysterious caliber, that for some reason, hits near the same impact point regardless of grains, very unique in all the world. It still ranks as "the one gun" most PH's would choose if they could have "only one gun" for all applications.
Like all calibers, it's a little the gun and a little the shooter and a LOT the load. A custom loaded .223 can take the head off a groundhog at 600 yards (we've done it). The key to cover a dime accuracy has always been custom reloading.
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