One thing.. It's LA. They loot in LA just because.
2nd thing is DURAI! A prison is already going to have a pecking order. If Durai wasn't the man, then he was near the top and probably killed his competition.
3rd thing is DURAI!
4th thins is DURAI!

Get where I'm going with this? If we've learned anything, it's that he's very organized and PROACTIVE. I believe that whatever ammo they got from the prison then Burt's store was used up offing the competition, the ones that didn't want to play ball and the zombies they encountered during the first couple days.
Remember the grocery store Angel and Saul hid in? They said it looked basically like a war had happened inside because off all the holes in the bodies.