First off, it depends on what standard you want to use for MOA values. In the late 19th century, it was 1/3600th of the range. At 100 yards, that's 1 inch; 10 inches at 1000 yards.
That all was based on USA smallbore and high power rifle target's scoring rings being spaced in exact inches. Virtually all the scope makers used that standard for their externally adjusted scopes. With a 7.2" mount spacing and the knobs 40 tpi threads and 50 clicks per turn, that moved the adjustment exactly .0005" per click, .002" for 4 clicks. 4 clicks equals one MOA or inches per hundred yards; 7.2/3600=.002. Aperture sights on target rifles with a 24" barrel typically had a 30 inch sight radius. 30/3600=.008333" or exactly what the 40 tpi lead screws on those sights move the aperture in 1/3 turn; three MOA per turn.
Along comes some engineers in the 1970's and 1980's and their calculators said there was 1.0472" per hundred yards for one minute of angle; their high priced trigonometry function calculators proved it. So what. It's been the "shooting standard" of 1 inch per hundred yards for centuries. But now it's confusing 'cause scope makers use both.
To say nothing of the four different world standards for a "mil" on this planet:
* 1⁄6400 of a circle in NATO countries.
* 1⁄6283.185 The “real” trigonometric unit of angular measurement of a circle in use by most telescopic sight manufacturers using (stadiametric) rangefinding in reticles.
* 1⁄6000 of a circle in the former Soviet Union and Finland (Finland phasing out the standard in favour of the NATO standard).
* 1⁄6300 of a circle in Sweden. The Swedish term for this is streck, literally "line". Sweden (and Finland) have not been part of NATO nor the Warsaw Pact. Note however that Sweden has changed its map grid systems and angular measurement to those used by NATO, so the "streck" measurement is obsolete.
Then there's the mechanics of the scope that move the tube with lenses that changes the target image position on the reticule. Whatever math they used typically only is close to their spec.
One should accurately measure their scope's adjustments on a ruler at 100 yards. Line up the scope very solid so it is well fixed in posision, then move one adjustment 40 clicks. If it's a shooters MOA adjustment, then the reticule had better move exactly 10 inches for 1/4 MOA clicks (or 5 inches if done at 50 yards) 10.472" if its a trig MOA scope. But it'll probably be someplace between 1 and 3 percent off. Darned few internally adjusted scopes are exact.
AT 999 yards, one shooter's MOA is 9.99 inches; one trig MOA is 10.4615 inches. Specs say its 1/10th mrad, but with its first focal plane reticule, it may be off as the front lens that focuses the target image there may not be exact for specs; most lenses vary a few percentage points. Which is why I suggest carefully measuring yours to see what it actually does.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master
Last edited by Bart B.; March 3, 2013 at 03:30 PM.