Abstract: A lock for a gun barrel has plugs insertable into the chamber and muzzle. The plugs impinge against existing stops on the barrel so as to limit travel of the plugs toward each other in the barrel. Once the plugs are in the barrel, a linkage connects them so that each stop prevents removal of the opposite plug from the barrel. Preferably the stops, such as the chamber diameter reduction and the muzzle choke seat, are inside the barrel and the length of each plug is coordinated with the distance to its in-barrel stop so that no part of the gun barrel lock is exposed to out-of-bore tampering. A lock prevents unauthorized disengagement of the linkage from the plugs. The gun barrel lock is of adjustable length and the plugs may have interchangeable diameter adapters to accommodate a variety of gun barrel lengths and calibers.