There is a small mix-up here between MOA and inches.
Here is the story: There are actually two "milradian" values, one is a civil engineering term, the other an artillery term, both are similar, but of slightly different values. In shooting, we will use the artillery value:
1 milradian ("mil") = 3.43775 minutes of angle ("MOA") = ~ 3.6" at 100 Yards
Almost everyone I know of rounds the mil value to "3.5 MOA", since it is indeed difficult, if not impossible, to resolve .06225 MOA with a rifle scope.
"Inches" are typically not used when viewing through a scope, since this introduces an extra math step, as well as some eyeball estimating. Why estimate when on can directly measure? We have a reticle in "mils", easily convertible to "MOA", why even fool with "inches"?
In fact, in an ideal world, one's turrets would read in the same unit of measure as one's reticle, and no conversions are ever necessary. We would have a mil reticle, and a mil turret, or, an MOA reticle and an MOA turret - what could be simpler than that?
1 mil is indeed a 1 mil, but, when compared to a reticle, KNOCKDOWN's point about the reticle's focal plane location is relevant and relates to the value of a mil at any given magnification *as viewed through the scope*.
Variable magnification scopes come in one of two reticle designs: First Focal Plane, or Second Focal Plane.
In a first focal plane ("FFP")scope, the reticle varies in size *with* the image as magnification is changed. This means that the reticle is still worth the same value at all possible magnifications.
With a second focal plane ("SFP")scope, the reticle remains the same apparent size as the magnification, and image size, varies. This means that it's value does not remain the same throughout the range. Most SFP scopes will be correct, and can be used to range targets, at their maximum magnification. Other SFP scopes will have an index mark on their magnification turret that must be aligned with the index mark on the scope tube or ocular housing in order for their reticle to be correct.
There are very real advantages and disadvantages to both styles, and even experts disagree on which is "best", quite possibly as a matter of individual use.
The equation to figure out your SFP scope reticle's mil value is based on the calibration setting, which is either your maximum magnification or the designated setting where the index marks align.
If you have a scope correct at 10x, and set it to 5x, then your target size is half value, or, the mil value is double, i.e.: a 1 mil graduation is actually worth two mils. So 10/5 = 2, and 2 x 1 mil = 2 mils
Another example, your scope is correct at 10x, but you crank it up to 16x:10/16 = .625, and .625 x 1 mil = .625 mils.
As one can see, this is rather complicated and it is much more simple to use even multiples of the designated magnification, like "half" and "double". For your question Zach, a scope designed for 10x, set to 20x, will render a target image at twice the size, and your reticle will be at half it's value. A 1 mil reticle (at 10x) will subtend 1/2 mil (at 20x).
In practice, one will usually wish to range using the highest possible magnification, which also happens to coincide with most scope maker's reticle calibration point. The better one can see, the more precisely one can "mil" and object, and the more precisely one can mil an object, the greater the precision rendered when ranging.
ETA: Ranging game with a reticle is not very reliable, due to the importance of exactly knowing one's target size.
The formula is "target size" (inches) x 27.78 / "mils" (subtended). If we measure a deer that we believe to be 18" from shoulder to brisket, and he is 1 mil high, our range would appear to be 500 yards (18x27,78/1=500.04).
If we are actually looking at a deer with a shoulder to brisket measurement of 15" instead of 18", our range is really 625 yards (15x27.78/1=625.05). If we had used our 500 yard drop value to compensate for this (overly long) shot, we will shoot very low, resulting in a bad hit or miss low.
