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In theory, open bolt subguns like Stens, MACs, and others with fixed firing pins are designed to detonate the round while the bolt is still traveling forward and before the cartridge is fully seated in the chamber. The forward inertia of the rather weighty bolt is supposed to counteract the rearward thrust of the detonating cartridge, allowing it to finish seating, then pushing the bolt rearward via residual gas pressure. (Or at least that's the way it was explained to me.) The bulged brass you saw was most likely the result of a weak recoil spring, allowing premature unseating of the cartridge case after detonation.
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