Geez Todd, what are you doing that your opened ammo doesn't last a month? It's not like the ammo is wrapped in plastic inside the box before you open it.

Anyway, nothing wrong with keeping it in a ziplock bag except maybe durability. A lot of military ammo around the world is stored in cans or heavy plastic "battle packs". I can't think of opening one yet that had dessicant packs in it.

I've got a heat sealer and a few thousand 3x5 4 mil bags. At first I just heat sealed a bunch of ammo for longer term storage. It turned out to be so convenient that now I heat seal all my ammo, new and reloads. The 3x5 turned out to be perfect. They'll hold (20) 5.56, (10) .308, (5) 12 gauge, and a plethora of other calibers. The convenience is great, I can throw a few packs in my pockets and go shooting. Never have to worry about sweat or dirt. The smaller size of the bags means I don't have a bunch of loose ammo left over when I'm finished. And the bags are TOUGH. I haven't worn holes in them while carrying them in my pockets all day. If there is a problem, it's that I have a hard time tearing them open.