Originally Posted by DJ's left nut
Kinda sounds like that dude's talking out of his ass.
Unless the upper actually has a 3-stage switch that gives you a full-auto mode at the up position, it's just an AR upper. Besides, an upper is an upper; steel is steel.
I mean I guess I could be talking out of my ass here, but as near as I can tell, there's absolutely no distinction between the upper receiver on an AR and an M4.
Still a badass gun and I guess a SWAT dude would know more than me, but if there's really some high-strength steel on an M4, that would be the first I've heard of it.
He said something about it having a larger throat/feeder for the round into the barrel.
Rock River calls the an M4 for some reason;