Mythbusters did a show once where they tested the heat tolerance of ammunition. They put the ammunition in an oven at 450 degrees until the ammunition finally popped and fizzled out. It didn't fire off like it would have had it been fired from a gun.
MYTH:::Bullets can explode with lethal force if they are stored inside a hot oven.:::MYTH
BUSTED
The Mythbusters placed a .22 caliber, .44 caliber, and .50 caliber bullet inside an oven. All of the bullets exploded once the oven was hot enough, but none of them were able to penetrate the oven. Without a gun barrel to contain and direct the propellant gases, the bullets did not develop enough speed to pierce the glass or steel portions of the oven. The shell casings actually caused more damage than the bullets.
Now in the event that you store your GUN (loaded) in hot temperatures
MYTH:::::A gun can fire a bullet with lethal force if stored inside a hot oven.::::MYTH
CONFIRMED
The Mythbusters placed a loaded .38 caliber revolver inside a hot oven pointing towards the oven door. Once the temperature was high enough, the gun automatically discharged and sent the bullet out of the oven, which could potentially kill anybody who happened to be standing in front of the oven.
So to simplify: It takes a lot of heat to make bullet go pop. I know lots of people store ammunition in vehicles that can get up to 120 + degrees without incident. Now if left in a hot atmosphere for long periods of time (1 month - years), the powder can break down making your ammo less effective.