>>1311There's a lot of arteries in the head, and he opened up several of them with the bullet, which passed through the nasal cavity creating a path for that blood to escape.
Even if the head was fully decapitated, the heart will keep beating for at least a few seconds thanks to the sinoatrial node (
http://en.wikipedia.org/wiki/Sinoatrial_node), and his heart rate was probably very elevated at the time.