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.
So the DC sniper dude was a fluke of mythical proportions? Most of the dozen or so people that caught one of his bullets are dead.
The heavier 7.62mm round would do even less damage at close range. It won't fragment as well as the 5.56mm, and the mass carries it through human targets. You can't rely on the increased diameter to get the job done, but the 7.62 gives you enhanced shoot-through capability. Ironically, the 5.56mm round excels at the closer ranges.
It's really logistics that's keeping the 5.56mm round in service. The US pushed its allies to adopt the 5.56mm round over the .243 round that the Brits wanted.