Well, I have taken a lot of "heat" for saying the the problem comes from heat. Most folks insist that either 1) the bullets expand sideways and burst the barrel or that 2) the compressed air does so. But experiments have been run with hardened steel bullets (which were measured before and after and didn't expand) and with obstructions that have had the middle drilled out so any air betwen the bullets could escape. The results were the same. When that energy dump takes place, the amount of heat is more than enough to cause barrel steel to act like a soft chocolate bar and stretch. And if it stretches enough it bursts.
The heat lasts too short a time to discolor the barrel steel or even harm the bluing, so even someone examining the barrel afterward won't immediately associate the problem with heat.
Also, and this is just MHO, I think that many of the Ka-booms in auto pistols are really due to barrel obstructions. And here is how I think it happens:
A shooter with two calibers of ammo, .45 and .40 or .40 and 9mm, gets the wrong round in a magazine, probably the top round. Most magazines will accept the smaller round and hold it.
The shooter inserts the magazine in the pistol and racks the slide. The small cartridge is put into the chamber but misses the extractor and momentum pushes it in far enough that it is not seen. The shooter thinks the top round failed to feed and racks the slide again. If he points the pistol down, the small round will fall out, but chances are he is trained to keep the pistol "pointing down range". So the second, proper size, round goes up the tube and is fired. Its bullet strikes the live round in the barrel; its inertia causes it to act as a obstruction, plus its primer goes off and blows the bullet and case out the barrel. Can I prove that any Ka-boom was actually caused that way? No I can't. But several statements I have seen lend some support to the idea.
Jim