I think it was with the M16A2.They had to to stabilize the heavier bullet they were adopting.Now they bullet stays point on and inflicts a small wound,especially at longer range.The short barrel on the M4 means the muzzle velocity is down to about 2800 fps.I’ll take my original M16A1 any day over the newer guns.Or better yet,my M1A.Now there’s a rifle.
Just about any bullet not designed to mushroom on impact will tumble at speeds above 2400 fps. That was one of the reasons the Geneva Conventions ban on exposed lead in bullets was moot; the higher speeds from smokeless powders and the advent of spitzer bullets allowed for nasty wound channels without the exposed lead.