Old 40s&w ammo having weak casing problems is well documented. That could indeed be the problem here. BUT another factor in the ammo being old could be that over time and use, chambering & unchambering live rounds could easily cause 40s&w bullets to be pushed back into the cases just enough to increase the pressure of the round when it fires. Less space in the case for the powder to combust in equals higher pressures in the casings. It does/has happened with 40s&w rounds. I don't keep rechambering unfired 40s&w rounds for this reason. After several re-chamberings I shoot the ammo at the next range session just in case. Just some food for thought...
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