The interrupter magnet is normally replaced by removing the
camshaft gear as the magnet is inserted through the back side of
the gear.
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Since in order for the gun to be effective it needed to be
behind the propeller, without the interrupter gear, bullets would
have destroyed the propeller causing the plane to crash.
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A "GFCI" is a ground fault circuit interrupter. For more
information see the link below.
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An example of a circuit interrupter would a fuse or circuit
breaker.