I first ran into the old saw about sighting in at 25 to be 3" high at 100 in one of my old Jack O'Connor books. The implication was that you'd verify it at 100 (and hopefully farther), but I'll bet fewer than half ever did. That 3" high business has probably saved the lives of a good many deer.
When I was still working, one guy in my crew was a captain in the National Guard. His unit did a tour in Iraq. We were taking about his M4 one night and he told me that they were instructed to sight in their AimPoints at 25 yards. I said, "You check them at 100 to be sure, right?". He said, "Nope. If they're on at 25, they're good." I tried to make the case that any error at 25 would be multiplied by 4 at 100, but he was having none of it. Still not clear whether that method was his or Army doctrine. During his unit's tour in Iraq, they only fired one round, that a single .50 cal fired at the place where they thought an enemy had fired from.
