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Very true. Old sg that become loose in the action will have a hdaspace issue. Excessive headspace often causes a backout rpimer @ any pressure level but certainly at a low pressure event.While I agree with the idea that most backed-out primers are due to low pressure loads, shotguns *usually* have zero headspace. In other words, in spite of low pressure, in most shotguns there is nowhere for the primer to go because the breach face is usually flush with the back of the barrel.
So, I'm gonna say the gun needs a look-see.
Or.... perhaps the action is opening up upon firing, which is a malady that many shotguns exhibit late in life.