A firearm trigger locking device with two sides and fixed hardened steel pins. The device is designed to be universal for a particular family of guns--e.g., all rifles, all shotguns, or all handguns (or all revolvers or automatic pistols)--of a given firearm manufacturer. The device further features a pin behind the trigger to prevent firing, hardened steel plates in each side sufficient in size to cover the trigger guard and prevent transverse application of a saw to the trigger guard, and a ring of hardened steel pins around the periphery of the trigger guard to prevent application of a saw between the plates. The method for producing this trigger locking device uses lateral and axial profiles of the family of firearms to develop features and feature locations that will allow it to be used with any member of the family of firearms.