Please don't tell my you've bought into this.
The 55gr M103 and 62gr M855 are ballistically stable out to 800+ meters. There never is, and never was, any "tumbling".
Within appropriate ranges (300m and less), the M16 rounds will fragment much better than the 7.62mm alternatives, causing significant damage.
The round flat out DOES NOT KILL THEM! Again we had several engagements where two burts of the M249-SAW did NOT drop the target. THAT is the wrong answer.
The tumbling I am talking about is inside the target, bouncing off bone, ricocheting through the next vital organ. The engagements are far too close for the current round to do this. The rounds have so much speed, they are zipping right through, not doing any damage but the pin prick it makes.