If you're using an ACOG, and have it zeroed @ 100yds, your bullet drop compensator is not tuned correctly, because that optic is scaled in meters. No net effect in most instances shooting at a human-torso-sized target from, say, 0-200m or so, but your shots beyond that will strike well below what you think, and the human trend is to blame the reticle, when it's the fault of the shooter that can't/won't RTFM.
100m is 109yds, if that helps; some folks don't grok metric. 27' of difference, practically meaningless on the front end of the arc, but a significant impact on the far end of it, presuming you want your BDC to be on.
Generally, the 50yd zero works VERY well for optics that set around 2.5" or so over center of bore. Higher than that (as in the above configuration), and the elevation angle of the barrel required to hit that 50yd mark is different from what it would be if you were using a standard irons/RDS sight height. You'll still have a no-shit 50yd zero, you'll just need to shoot to see where it recrosses you LOS on the downward end. It'll probably be further than the 225-255yd mark usually achieved, and the greatest height of the round over LOS ought to be greater.
Your ACOG will be somewhere around 2.755", model dependent, so whatever RDS you have on top will be at least an inch higher. Inside 50yds, it means you'll have a larger sight to bore relationship to remember, too. So, height over bore doesn't affect your initial zero, as that's something YOU establish, but if you punch it in to one of the ballsitic calculators, it'll give you a reasonable read on where it will hit on the far end.
Last edited by JSantoro; 01-20-10 at 19:43.
Contractor scum, New & Legacy Equipment Trainer
PM Infantry Weapons, USMC
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