I need a little help...

So, I've gotten to the part where I have to fight Barret, but the big problem is that all I have weapon-wise are a rifle with 11 bullets, a sniper with 5 bullets, and a grenade. I used it all on him, shooting him in the head and all, and the son of a bitch didn't even flinch. After I'm out of ammo, I obviously die. Any suggestions on how to pass? Also, I don't have earlier save games. I over-write every time.

Thanks!