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.
Yep...I have an early 1970s Colt SP1 with that slow rifling.