Yeah, fat 50s are great, I have a few, including "red" ones for my first aid gear (with the white cross)

If you're using them as a tool box, throw a couple of moth balls in the bottom. Believe it or not, the stuff they outgass helps prevent rust! Also add some silica gell packs. Remember, that if the seals are good, ammo cans are water resistant (NOTHING is water proof)