Rather than waste time worrying this to pieces, with one foot on the banana peel, go down to Walmart. They have, usually, several AR15s, under $900, that are perfectly serviceable. Bushmaster is fine. So is DPMS. You can mount any optic you want on just about any AR. Do it.
If you want to shoot...shoot...don't talk! Tuco
USAF Munitions 1969-1992