As I just posted in another topic.....
"What a bullet destroys is a matter of shot placment, not so much bullet construction." Or in this case, substitute bullet construction with "size, velocity, and energy of the projectile."

That .375 is more than enough gun, with proper shot placment. It is not that your son did not try to do a good job, he just suffered the concequence of using someone else's gun. He found the 2 voids in a buffalo that when hit, do virtually nothing. As they say, " it happens"