It’s a distinction without much of a difference, though. Apart from auto and burst fire, a modern AR-15 does everything an M4A1 does. The Marines’ M4 and M16A4 models don’t even go past burst.
If semi-auto rifles are going to be legal at all, they should have a small integral magazine that’s non-trivial to modify. The sheer efficiency of these rifles makes them really good for assaulting humans, because that’s what they were designed for.