Actually, I've shot at 1000 on several occasions at Camp Perry. I understand what you are saying, but we are talking theoretical, and in some instances, rifles shoot BETTER at ranges beyond 100 yards. A 100 yard group is just a measurement. I've shot on almost windless days, and that sure helps a lot. Will a marksman always hit a target at a reachable range, like 450? Naw. Throw in stress, a micro-movement of the body, a gust of wind; there goes the mini-group. But, a man sized target is just a tad bigger than a POSSIBLE group from a stock carbine. Could a guy hit 50% of the time at that range? I'd be happy with that. In combat, even 1 out of 3 shots would be great, especially if it was shot #1.
I recently shot a 200 yard group with my GI Saginaw SG carbine that came in under 3". I'm sure it won't happen all the time, but it is possible, on occasion. That is why I have faith in PROBABLE hits at longer ranges with the carbine. I have a lot of "holdover" shooting in, with such guns as 300 Whisper and subsonics, and even the 510 Whisper (also subsonic with 600+ grain bullets) at 600 yards. Not sure of possible groups at that range, as it was informal shooting (with API bulets, which will let you know where you are hitting on rocks.
). Of course, in combat conditions, target are obscured, ranges are unknown, the shooting is fast and furious, and you don't always wait to see if you have a hit. It is still interesting to practice the long shots, and your range estimation and performance are always better as a result of it.