Blow-back operated guns do not actually lock the breech.
They rely on the mass of the breech block to delay cycling till pressures have fallen to a safe level.
Depending on what in the action stops the breech blocks forward movement a partially unscrewed barrel would result in light strikes.
The firing pin does not protrude much on a rimfire, so all it is doing is pushing the shell further into the chamber and when the shell stops the pin does not have enough momentum left to ignite the primer.
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