I just got around to reading some of the 'extra' paper that came with a Hi-Point carbine last year. "Hang fires and how to avoid them". I'm going to just quote the main text, because there is nothing to skim over. "It is generally accepted that commercial ammunition (no matter how old) is high quality and cannot fail. Experienced gun people know better. Any ammo can be 'spoiled' by subjecting it to changes in temperature. These fluctuations cause moisture to enter around the primer pocket and condense in the powder in the flash hole. This is dangerous for semi-automatic and full-automatic weapons because when the primer is set off, instead of igniting the powder charge, it is interrupted by the wet powder in the flash hole. The primer has enough back pressure to open the slide or bolt and then the powder charge is set off with the breach open, releasing great pressure inside the rifle or pistol. The result can be damage to the magazine, frame or even the slide or bolt. Nearly all of this can be avoided by simply shaking the ammo to move the damp powder out of the flash hole, allowing the cartridge to function normally. SHAKE YOUR AMMO!" Damp powder in the flash hole. Shucks, that goes back to flintlock days, but I never made the connection.