Abstract: A button lock for a depressible magazine release button bolt of a lower portion of a firearm of the type having a selectively removable upper portion includes a body having a top surface, a bottom surface, and at least one peripheral edge. The body includes a main aperture traversing the bottom surface and sized to accept a screw insert therein that itself includes a threaded bore for receiving the button bolt. The body includes at the top surface thereof a laterally-projecting extension terminating at a downwardly-projecting lip. The button bolt is engaged within screw insert in the main aperture. With the extension positioned over both the lower and upper portions of the firearm, the downwardly-projecting lip prevents the button bolt from being depressed when contacting the upper portion of the firearm. A permanent plug may be fixed within the main aperture to prevent removal of the screw insert after installation.