We had some steel stop plates that we used for Practical Pistol competition - .45 and 9mm just splattered, stopping the timer...
We plinked them one day out of boredom from ~150 yards with ARs and were surprised as hell to drive up and find them looking like swiss cheese.
Small holes, to be sure, but drilled all the way through!
Drilling little holes is all the 5.56 is good for nowadays. At least as it comes out of the current generation of M16/M4’s.
I’ve just finished reading “Sniper Elite” by former Aussie SAS sniper Rob Maylor. He recounts one incident where his buddy drilled a Taliban sentry three times with an M4 with no visible result. Fortunately Rob instinctively double-tapped the guy through the brain pan. The first stories about this phenomenon coming from the Blackhawk Down debacle in Somalia.
I remember quite well reading in SOF years ago a reference by the late machine gun guru Peter G. Kokalis that the then-new M855/SS109 ball round for the M16A2 (mid-1980’s) was overstabilized because the rifling twist was tightened to stabilize the longer tracer bullet, thereby destroying its ability to “yaw” in soft tissue and make icky-poo splatter.
That might or might not be the explanation because the old M193 bullet of Vietnam fame fired form the AR15/M16/M16A1 was a legend of lethality. There were many colorful (oog!) stories to come out of that conflict as to the spectacular demolition caused by that bullet.