One of our club members is retired US Army Light Col (Infantry), three tours Vietnam. We've often discussed this issue, back then it was an M-14 V M-16 discussion. Without reservation, he favored the M-16. Much lighter, easier to carry AND lots more ammo. As to the M-4, I know they've ahd issues with slower MV with the shorter barrel when it comes to penetration, in fact, we had a significant thread on this in the past.

A good principle to guide me through life: “This is all I have come to expect, standard lackluster performance. Trust nothing, believe no one and realize it will only get worse…”