I know a guy who left a loaded gun in his oven, forget it, and turned it on to 275. An hour later, he remembered, but none of the rounds had cooked off. The rubber grips, however, were kinda....gooey.
On the other hand, I have heard of a police chief who suspended himself after he left HIS gun in the oven, apparently it was over 300+ degrees, and it blew several holes in the oven, and destroyed the gun as well. But I have kept a hundred rounds of spare ammo in my patrol car, as well as my personal vehicle, for years, without any problem. And it gets HOT around here.
As for rounds in a campfire, they WILL go off, but once they get hot enough to blow, they pretty much just burst. That's not to say they aren't dangerous, but less so than a bullet from a barrel. But on one campout years ago, a drunk threw a box of shotgun shells into a bonfire, and we all bailed, and hid out behind a truck until they had all gone off. Several tents got ventilated by flying debris, and a truck got some paint chipped off. The miscreant who educated us was summarily dumped from all future activities involving fire and guns!
Papajohn
