I'm guessing brisket or partial brisket. Get some blood but not a lot and it'll stop real quick. Possible you clipped a lung but I really doubt it. And as far as mechanical vs. a fixed, we all know the mechanicals do loose some punch. He had a light arrow, moderate poundage, smack some bone and you don't get what you want. Although in this case, it sounds like it was too low to do the deed with any head.