In the SIG P320 armorer’s course, the instructor mentioned that SIG engineers designed the P320’s extractor to blow out and relieve excessive gas pressure when a case ruptured and vented the pressure into the pistol, thus preventing further damage. That is exactly what happened. A 9mm brass case ruptured, allowing the gas to enter the pistol; however, the only damage my pistol suffered was the loss of the extractor (I could not find it).
Why did the case rupture? Typically, there are several causes of case ruptures including too much gunpowder, a weakened case (from too many reloads or poor manufacture), or a chamber that does not fully support the case. I was shooting my reloaded training ammunition which opens the possibility that I had too much gunpowder in the round. I do not believe that is what happened, however.
I am very familiar with the signs of overpressure in ammunition. The primer from this round exhibited no signs of excessive pressure. The case in question was a brass case from the Tula Ammo cartridge company which had only been reloaded one time. Since the primer exhibited no signs of excessive pressure, I can only conclude that the case itself was weak.
In semi-automatic pistols,
the case usually ruptures on the underside where it does not have
complete chamber support because of the feed ramp cutout. This is where
the case ruptured in this instance.
Excessive pressure flattens the primer and can cause the firing pin to pierce the primer. With extreme overpressure, the pressure blows the primer out and destroys the brass case. Case ruptures often result in magazine damage with the magazine blown out of the gun and may also damage or destroy the frame of polymer pistols which means the pistol must be replaced. In the example shown below,the case completely ruptured and vented all of the pressure into the pistol. The pistol was a Springfield XD and the overpressure damaged the magazine, damaged the top round in the magazine, and completely destroyed the pistol; however, it did not injure the shooter.
In my incident, I believe the SIG design turned what might have been an expensive event into a minor annoyance.
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