A firearm includes a frame and a firing mechanism having a sear (e.g., configured for controlled release of a firing pin) and a trigger bar for actuating the sear. The trigger bar is disengageable from the sear. A locking mechanism is attached to the frame and can be actuated (e.g., by using a key) to selectively disengage the trigger bar from the sear. The locking mechanism may include a pin extending through the frame and axially rotatable in place. A first end of the pin is accessible from the exterior of the frame for key actuation, and a second end of the pin is positioned proximate to the trigger bar in the interior of the frame. The second end of the pin is provided with a cam surface for laterally shifting the trigger bar out of engagement from the sear when the pin is axially rotated using a key.