I read a really interesting article in a 1980s American Rifleman about a credible experiment in cooking off ammo to see what happens. The author built a 4'x4' room with sheetrock walls, used a fireman's jacket as a target, and cooked off individual rounds with a propane torch. Between the cartridge and the torch was a chronograph. He used a variety of ammo, including centerfire rifle and pistol, 22LR, and shotshell. He found that the bullets can fly at up to about 250fps, but the primers will fly up to 900fps (even though they only weigh about 2 or 3 grains.).
The bottom line is that firefighters needn't be overly concerned with loose ammo cooking off because they are adequately protected by clothing and face protection. But looking at the velocities, I wouldn't want to be anywhere near a campfire that had cartridges thrown in it.