Interesting account years ago of one of our American friends suffering a house fire, which basically destroyed his home. Rooting through the debris with a few firefighters, they uncovered a twisted ammo box, in which he had stored a loaded revolver in a cowboy rig, with extra rounds in leather cartridge loops on the belt.
The round in line with the barrel had discharged, and gone through the ammo box, and struck the basement wall, dropping to the floor. The rounds in the rest of the cylinder has also cooked off, and I don't recall what he said that did to the revolver. Rounds loose in the ammo box just cooked off, and left no dents or bangs on the inside of the ammo box. But, the rounds in the cartridge loops on the gunbelt that had been nicely wrapped around the holster, left a nice distinct circular pattern of holes in the side of the ammo box. It seems the cartridge loops added just enough support to the case walls to cause the cartridges to actually discharge, rather than simply burst.
Probably not much risk from cartridges loose in a pack discharging from rattling against each other. A fellow at Dillon Precision, answering my question about how to get rid of case lube after progressive loading .308 cartridges, said he just tumbled them in corncob. I squeaked "What, you tumble live cartridges? And they don't explode?" He kinda laughed, and said none of the thousands of rounds he had loaded and tumbled ever did.
I never have, mostly because I only have one tumbler, and I don't want to get the tub all gooey with lube, and then try to clean brass, but if I get flush, and can afford a new tumber, I probably will follow his advice. Dillon's crew has never led me astray, so I figure their advice is good to go.