An M16-type automatic rifle is provided with an improved gas operating system. The tube extending forwardly from the bolt carrier key is of such length that it permanently mates with the aftwardly extending gas transfer tube as the bolt carrier key shuttles forward and aftward in operation. This arrangement minimizes fouling by combustion products from the firing of ammunition. Modifications to the gas transfer tube and barrel nut assembly accommodate the extended bolt carrier key tube and facilitate quick change of the barrel in the field with a standard tool.