Unless ammo cans have changed drastically in the recent past, they don't utilize a non-gasketed slip on lid. As I recall from the last time I opened one, (about 5 minutes ago), ammo cans utilize a high pressure compression gasket incorporated into a rather durable lid. As such, I would rate them as quite a bit better at repelling moisture than what is, essentially, a mini cookie tin. No container will keep moisture out if opened during a hurricane, but ammo cans will keep moisture out in all but the most drastic scenarios. A metal tin whose lid simply squeezes in place probably can't make the same claim. For an arid environment, I imagine those spice containers would do fine, but if your state has more than, say, 1% average humidity, I'm thinking they might not be adequate even when tucked out of direct contact with rain.
I am curious as to why no one seems to use a dissecatent pack for absorbing moisture in some of these cache containers?