the arsenal they raided

It's just typical TWD writing. We're expected to believe they've lived in specific area for years when food and other resources are low and they just happen to find these places they weren't aware of before.

In one of the filler episodes from last season, the one with Daryl fucking about with his bike, he came across one of the soldier zombies with a couple of mags in it's pocket/vest. I guess the writers just assumed that the viewers would assume they just hunted around the area until they found the base.

I'm surprised they didn't have a way to wipe them zombies out easier. I mean they where all mainly gathered together so something like fire possibly could of killed them all quickly and allowed them full access unless they could have set fire to the food.