Things that can cause excess pressure are oil in the chamber or excess grease in the barrel. It usually won’t make a gun blow up, but excess grease in the barrel can cause a bulge and ruin a gun. I know a person that always put way too much oil in his chamber after cleaning it. We were shooting one day and upon him firing his 7 mm magnum, smoke came out from the chamber. The bolt was hard to open and the case was stuck in the bolt head. We got it out and the primer pocket had expanded. Upon cleaning his gun the problem disappeared. Excess oil makes the chamber smaller, preventing the brass from gripping the chamber walls as normal and increasing back thrust. In a weaker gun, it could be destroyed.
Old ammo that is stored in a hot place for a long period of time may get stronger and inconsistent. It may not lead to where a gun blows up, but could cause pressure signs and fire very inconsistently. If you have old powder that shows a red dust, dispose of it. It is going to be unstable and possibly dangerous. While it is a rare occurrence, a sharp firing pin or firing pin that is too long could pierce a primer releasing gas in the action. In some cases, it could damage a gun or injure a shooter, especially in guns that don’t have good systems to vent gas. Shooting glasses should always be worn.
Poor quality guns can blow up, even with normal ammo. There were a couple of companies in California that went out of business because of their guns blowing up. I have personally shot a Davis Derringer in 38 special that blew up in my hand. Using regular ammo, the barrels blew off and cut my finger. We looked for the barrels for hours but were never able to locate them. I was lucky to get off so lightly in the injury department. I examined another one and found that the barrels had thin steel inserts with the rest being aluminum or some pot metal. No wonder it blew up.
I have another Derringer that I obtained from a customer in 45 Colt that blew out the side using normal factory ammo. For the record, I don’t like Derringers, even good ones, but that is my personal preference. In truth, quality Derringers are perfectly safe. I also have a Davis 380 that let loose. Upon examination, it showed that the gun fired before the breech was closed. It put a tear in the bottom of the case, which blew gas throughout the gun and dismantled it. The customer was not injured but he received some spray from the gas. It always pays to spend a few extra bucks and get a quality gun for safety and reliability. This applies to long guns as well as handguns.
Read "Why Guns Blow Up Part I"
Read "Why Guns Blow Up Part II"
Read "Why Guns Blow Up Part III"