A gun barrel adaptor includes a body having a central bore therethrough and including a flange, the flange having a hole therethrough, the body having a lower side on one side of the flange and an upper side on another side of the flange; a flat spring attached to a lower side of the flange; a stop disposed in the hole in the flange and extending above an upper side of the flange, the flat spring being operable to resist movement of the stop towards the lower side of the body; and at least one locking tab disposed on the upper side of the body. The gun barrel adaptor provides for quick change modularity of muzzle devices without the use of any special tools.