In the Guardian Caves, (Map Marker: Guardian Peak 3.20 on the eastern side of the map), you will come across a hurt NCR trooper named Halford. When you talk to him, heal his broken leg (either by your medicine skill or doctors bag) and talk to him about his service rifle and he'll offer it to you if you help get him out of there alive. Then you need to convince him to help you clear the rest of the cave of lakelurks. After dialogue is over and he gets up, run towards the entrance you came in, NOT the exit to the lake. When you get close, if your not in combat, he will initiate conversation and say hes cant help anymore and he has to leave. Choose the dialogue option "But you said you would help me clear the caves" (or something like that lol) And then remind him about him offering you his rifle, he'll give it to you along with 15 AP 556 rounds(and the Dog Tag Fist schematics), but the option to ask for his gun is still there and can be chosen again. I was in a hurry to explore more so I only did it twelve times, but it never broke. I'm pretty sure that you could keep going with and fill up yours and any companions' you have inventory(s), if not for the rifles then for the ammo.

This was a few minutes ago, so I might not remember exactly but it shouldn't be that hard to recreate. Again, this was on the xbox 360, I have no idea if it works on either PC or ps3, but you can try it.