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When you fire the proper cartridge in a gun, the gun stretches a bit but returns to normal. This is called elastic deformation. Elastic deformation is what a rubber band does in normal use. Pull on a rubber band, then release it, and it goes back to its original size. If you fire a cartridge that is a little too powerful in a gun, the gun may stretch but not completely return to normal. This is called plastic deformation. Pull on a Tootsie-Roll and it stays pulled; it undergoes plastic deformation. Most of the time, guns are like rubber bands. However, if you fire a cartrige that is too hot, the gun turns into a Tootsie-Roll.

All guns, even the best made guns, have microscopic cracks in them. Normally these cracks don't matter, but stretch the gun too much and the cracks open wider.

If you look at the loads placed on a barrel during firing, you will find that the force to pop the front of the barrel off of the rear of the barrel is only one half of the force to split the barrel. This means that when a barrel splits, the crack runs down the barrel, not around it like a ring. (Ringing in a barrel is caused by a different phenomenon.) The stress that causes the crack to run along the barrel is called "hoop stress".

This exact same phenomenon is why water pipes split open with the crack running along the pipe. Look at a water pipe that freezes and ruptures and you will notice two things: 1) the crack runs along the pipe and 2) the region around the crack is bulged out. The bulging out is an example of plastic deformation; the water pipe was streched and didn't go back like it was.

Barrels are designed for a cartridge of some specific maximum pressure. As long as you keep the pressure below that maximum, everything's hunky-dory. Fire a cartridge of too much pressure and the cracks begin to open up. Due to geometry the stress on the inside of the barrel will always be greater than the stress on the outside of the barrel. Barrels may last tens of thousands of rounds with normal pressure loads. If they are subjected to a load that is too powerful, they may fail within a half dozen rounds. Sometimes, like the in case of a double charge, the barrel may fail on the first round. When a crack spreads far enough, your gun suddenly turns into a grenade...this is bad.

Even the strongest guns will fail under too much pressure. Glocks are among the strongest pistols on the market, yet I've seen them with their chambers peeled forward from reloading mistakes. If you want more power, get a gun that is designed to handle it. Remember cracks kill. ;)
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