I do it two ways with the AR, the way I suggested before, using the known zero iron sights and verifing the scope coincides. I do this on guns that have see through sights. Works pretty well. The other is to put the upper on a rack or something solid so it doesnt move, pick a point about out in the yard somewhere, or put a target out there, something you can see easy. Line the target up in the bore of the rifle, then, without moving the upper, look through the scope and see where its hitting. Just dial it on to the target so when you look down the bore and see the target the cross hairs are in pretty much the same place. I usually try to shoot at 25 yards first to make sure I didnt screw up, but 90% of the time, I'm real close. This works for bolt guns to. For guns you cant see down the bore, like an M1A, etc. I use one of the 90 degree bore mirrors. This make sense?