If you are doing"slam fire",...
you have a problem as the recoil lock is not engaging, during the ignition sequence and the full recoil impulse is slamming into the receiver.
The lock is the long bar on the lower left side of the carrier, it fits into a corresponding cut in the receiver wall. Its either broke or gummed up or the cut is not cleared.
They are good shotguns, mine retired to HD use.
Look on gunbroker.com, you'll occasionally find mag tube there.
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