No ammo "tumbles". It would be impossible to fire "tumbling" ammo at anything with any kind of accuracy. The Tumbling-ammo myth is often attributed to 5.56/.223 AR rifles by the gun control crowd as evidence as to how deadly they are, when the truth is that they are weak compared to other calibers. They are so weak that some states don't even allow them to be used for deer hunting.I don't disagree, but from the picture it looks as if it passed through the bicep. Often at very close range ammunition doesn't expand so you can get a clean pass through. This is one of the reasons SOCOM developed the .458 SOCOM - they found when house clearing at close range, 5.42 passed through causing little to no damage (although worth mentioning military ammo tumbles rather then expands). My best guess here, is as you said the energy. It ripped the muscle away being a relatively loose piece of the body muscle attached by a tendon at each end in a casing of skin rather than explosively removed it. I rather suspect had it hit the torso in a non vital area and missed bone, and thus hit a more supported area, it would have been a pass through. As you also said though, play dumbass games, win dumbass prizes and he won a doozy, disability for life.
I've put many hundreds of rounds through my AR. All the paper I have ever shot had nothing but beautiful little perfectly ROUND holes in it.