military ball ammo incapcitation is largely attributed to the upset, or "yaw" condition that happens as it passes through the body (basically the center of mass is positioned nearer the base of the bullet, and so the disruption caused by impacting tissue/etc cause it to yaw into a kinetically stable attitude)... there are a number of variables at play here, (geometry, anatomy along path, bullet construction, bullet mass, bullet impact velocity, etc), any of which can result in a dichotomy of effects... w/ 5.56, the bullet must pass through about 12cm of tissue before it begins to yaw, ...which produces a large permananent and temporary cavity - this is what gives it maximum incapciation effect,... in malnourished individuals, i.e. "skinnies", ...this effect may never fully develop if it exits before upsetting...
here's one good read by fackler;
http://ammo.ar15.com/project/Fackler...ary_rifles.pdf