It's an M4, but I think you can actually have M16s with single, burst AND auto modes. But M4s are the main auto weapon, M16s are designed for long range battles, where you need slower rates. They also have a cover for the muzzle when you attach an M203, and an extra foresight. So HA!
BTW, SA80s pwn M16s
Or not. SA80s suck, but they look cooler.