Looked like a burst from an M16A2. Or a quick burst from a SAW. You can see a couple puffs hit the ground just before old boy takes one in the grill. Since the first shot on a burst is usually the one that hits, I'm guessing machine gun, not M16.
Could be a 7.62mm, but I think his entire head would have disappeared if it had been 7.62, so I'm guessing SAW and sticking with it.
It's the new TV sensation, watching enemies of your country get slaughtered. Beats the heck out of anything the networks have on for fall......