mile, with a scope, can, AR-15, and forend mounted bipod, slowfire. Such shooting can be learned by a teenage girl, in one day. Somebody with a bit of desire, who's never been exposed to blasts without ear protection, or recoil, can learn to get such hits in a very few hours. It's mostly the blast and recoil that cause learning problems, and the 223 gas op auto, with a can, has none of either. But it can easily have the same effectiveness at 1/4 mile that a 4" 357 Revolver has at 20 ft.
Longer range hits start being LUCK, in many ways, regardless of who you are or what gun or load you use.
So it quickly means 10x as much work and expense, for 10% increase in ability. Why bother? U can get within 1/4 mile of ANYBODY, and you are SAFER at 1/4 mile, with a canned auto rifle, than you are a 1/2 mile, with a noisy bolt action, for certain. You are many times more likely to get the hit with the canned AR at 1/4 mile, too. If you don't HAVE to get the hit, then why bother with the shot at ALL, hmmm? If you don't have to get the hit, you dont have to take the shot, and that means you SHOULDN'T take the shot, as a civilian survivalist.