Abstract: A gas chamber fixture having attachment to the barrel of a gas-operated weapon wherein the barrel and the weapon are mechanically attached by means of a mating mortise and tenon. A bead is received in recesses formed in the adjacent surfaces of the mortise and tenon to facilitate rotation during attachment and provide sealing engagement of the joined parts. A gas passage between barrel and chamber is provided through the bead to provide the driving force to the drive mechanism located in the chamber.