The 7.62 X 39 round does not like the strait mag well of the AR15 overly well. The AR47 setup works because it uses unmodified AK mags.
I have also seen AR's .204 Ruger too.
"They say you can't use a rifle or shotgun indoors because a bad guy will grab the barrel. Yeah? Well, he better hang on, 'cause I'm gonna light him up and it will definitely be an "E" ticket ride."
"Make (your attacker) advance through a wall of bullets. I may get killed with my own gun, but hes gonna have to beat me to death with it, 'cause it's going to be empty."
Clint Smith, Thunder Ranch