I have see someone shoot a 3 gun with a 9mm MP5 clone, and did very well at it. As long as you are comfortable shooting the M1 than go for it. Pratice reloading the rifle on the fly, the AR platform shines in the reloading speed catagory, due to the layout of the mag release bolt release.
"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