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An NBB works in a similar fashion except the knocker pushed the gas valve open (inertia) and gas pressure and a return spring on the valve allow it to close again.
The main difference with a GBB is that in a GBB the knocker is being held in place until the action cycles.
In both cases the gas will vent when pressure drops too much (empty mag, cold weather) but for the GBB its because the action can't cycle and for an NBB its because there isn't enough pressure for the valve to close itself.
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