Given the chunk of meat blown out of the pistol wielding assailant's bicep later on, I'm pretty sure that if he had shot the first guy in the head with his rifle it would have made a more immediately fatal injury. Can't say about the caliber.
First guy died in less than a few minutes - shot hit side of skull taking a piece with it
With the way the first guy's head was wrapped with a shirt, it's hard to see just how much damage was done. There might've been a pretty horrible exit wound, but headshots can be unpredictable. Either way, he looked pretty limp when they toted him to the SUV.