To clarify my post #19 for those unfamiliar, the 37 flexes and applies rotational torque to the extracted shell, such that when cycled rapidly the rim of an ejected plastic shell may strike the exposed primer of the round in the magazine tube. There were miles of high speed motion picture film documenting this, which happens mostly in 16-ga. and 20-ga. guns when firing plastic shells. Much less often in 12-ga. There were multiple lawsuits about this. At Ithaca in the 1970s they had an old Army wall locker full of guns with blown magazine tubes.