Tumbling terror.
It was always my understanding that the slow rifling (1 to 12 I think) of the early M-16 and it’s prototypes would just barely stabilize the (then) 55 gr. bullet...thus giving it the propensity to tumble when striking an adversary. The trend has always been to faster rifling to stabilize heavier bullets and give better accuracy.
I have said what the Dr said since 4/26/69 when I was shot twice. Had two 5.56’s gone through the same places in the same leg-I would not have a leg if alive.