I have seen several torture test videos on youtube and the metal ammo cans are MUCH better in this regard (ie. waterproof) than the cheap plastic Wal-Mart ammo cans.
I was talking to the guys at Georgia Arms (big commercial reloader) about this. The metal cans just aren’t as available surplus as they used to be. That’s why so much of their stuff is in the newer plastic ones, even though the pictures at their web site still show the metal ones.
http://www.georgia-arms.com/
I should buy some more at Costco . . .