I agree with Fly Navy. I don't much buy into stopping power.
You can shoot someone in the chest with a .22 short and kill them if you hit just right, conversely, you can pepper someone in the chest w/ 7.62 and they might live until they bleed out...
It is all about round placement, not round size/round energy
In my opinion, when you are talking about rifles that can penetrate body armor at 600+ yards, stopping power, especially for MOUT conditions, is a little overrated.
As FlyNavy said, it is mostly all "he said, she said." You could fire a thousand rounds and kill your intended target in one shot with all of them, but let Superman take two rounds to the arm and not die, and everyone screams "stopping power!!!"
I buy into the "stopping power" argument a little more when talking about pistols, notably the difference between the 9mm and the .45, but IMO, nearly all rifles rounds are going to have enough muzzle velocity and energy to be effective at stopping a man-sized target.