For those of you who have trouble understanding the difference between MILs (milliradians), inches and M.O.A. (minute of angle), I'll offer the hillbilly explanation.
An inch is, well, 1/12 of a foot. An inch at 100 yards is the same size as an inch at 1,000 yards. An inch is a linear measurement.
MILs And M.O.A.
MILs and M.O.A. are angular measurements. This means that as distance increases, so does their size. At 100 yards one MIL equals 3.6 inches and one MOA equals 1/60 of a degree or 1.047 inches. At 200 yards a MIL or M.O.A. is twice as large as at 100 yards, and at 1,000 yards it's ten times as large.
How Does This Work?
Most riflescopes have M.O.A. adjustments and each click equals 1/4 of an M.O.A. Let's say you're shooting at 400 yards and need to compensate for 25 inches of bullet drop, which equals 24 (23.87) M.O.A. Four clicks on the elevation adjustment equal one M.O.A., so how many clicks would that adjustment require? Ninety-six (4 x 24)? Nope. You see, at 400 yards, one M.O.A. is equal to 4.188 inches (4 x 1.047), so each click would equal 1/4 of 4.188. The correct answer is 23 (22.92) clicks.
With MILs it's the same ... just different—and here's where it can get complicated. At 400 yards, one MIL equals 14.4 (4 x 3.6) inches. If you have a MIL reticle, but each click on your elevation adjustment equals 1/4 M.O.A., the correct adjustment is still 23 clicks. Some scopes with MIL DOT reticles have 1/10 MIL adjustments. At 400 yards, 1/10 of a MIL equals 1.44 (14.4 / 10) inches. In that case, your correct adjustment would be 17 (25 / 1.44) 1/10 MIL clicks.
Don't be ashamed if you are. Most of us were raised to understand inches, and it's difficult for us to think in terms of angular measurements. This is why ballistic reticles have become so popular. With ballistic reticles, shooters don't have to do a math problem before they pull the trigger. Ballistic reticles work because the additional aiming points subtend to—equal—a bullet's drop in inches at certain ranges. Essentially, a ballistic reticle lets you hold over the correct amount of inches to get a hit at a certain range.
However, it will be necessary to fine tune your zero with a ballistic reticle to make sure the additional aiming points correspond to your bullet's drop from your rifle. Just follow the zeroing instructions that come with scopes equipped with a ballistic reticle. Why will the drop be different from different rifles? Ammunition and muzzle velocity variations, the height the scope is mounted above the bore of your rifle and atmospheric conditions you are shooting in all affect a bullet's trajectory.
Does Any Of This Matter?
Well, yeah. When you're shooting at long range, everything matters. Of course there's nothing wrong with memorizing your bullet's drop at specific ranges and then holding over the target a corresponding amount. What's actually more important than how you make the correction is that you know the exact range to the target. At ranges of 400 yards and beyond, an estimation error of 25 yards will cause you to miss an 8-inch target—with most rifles—regardless which correction method you choose.