I tell my customers if the sub sonic ammo shoot 1.5" or better at 100, then that's good. I have shot a few 1/2" groups with a tuned sub sonic load, though, so they can be made to shoot better. Shooting out of a bolt gun is easier to get the accuracy. AR's you have to use the right powder to get it to also cycle. Subs are a heavily reduced load to make a certain speed, and can be a little finicky on what they like. You are taking an efficient round, and making it inefficient as a sub load. There are a few tricks to getting them to work better, though.