If you look at the depth of penetration and then take a tape measure and measure the thickness of your upper arm, thigh, and upper and lower torso you might note that some people are not thick enough for rifle rounds to tumble as the rounds pass through them.
Even 5.56mm on a solid torso hit may just zip through if the torso in question is skinny enough.
It would be nice if we all could go to the store and buy magic bullets, or maybe disintegrator rays...
Any particular shot placement can be awfully difficult to achieve as well, so I guess you turn your target into a colander if that is what it takes to stop him.