A lock for a firearm with a grip safety, and a sear engageable to a biased hammer in a cocked position and releasable by a trigger is disclosed. The lock has a housing defining a first bore within which a mainspring that biases the hammer is received, as well as a second bore. There is a locking pin retractable into and extendible out of the second bore of the housing. When the locking pin is in an extended position, the grip safety is restricted to an engaged state, blocking movement of the trigger. An actuator disposed in the housing and cooperatively linked to the locking pin provides the motive force for retracting and extending the locking pin.