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Hmm, thats a good point about the temperature variations, but how dangerous is it to keep tens of thousands of rounds of ammo in the house? I don't want the house to catch on fire, and then the insurance company deny any claims because the ammo was in the house causing a fire.
If the house burns that bad, then everything above the basement will probably already be history anyway. It would make for a 4th of July type event with the firecracker type popping that will come from the hole in the ground.
As long as you store it according the the law (where applicable) then there shouldn't be any issue. Check with the insurance company if you intend to store vast amounts of loaded ammo. You may want a rider of some sort to make sure it is covered if you have that much.
I have about 10k of factory ammo in various calibers, but probably 30k worth of bullets, matching primers and cannisters of powder. My rider covers all the gun stuff for what it should be worth. I may have to up it with ammo prices being what they are right now.
I would be more worried about any black powder you may have around. Smokless powder is a propellant whereas black powder is an explosive. Stored improperly, you could have one helluva bomb on your hands.