Abstract: A self-contained self-muffled pneumatic gun for dispensing one or two combinable flowable materials from a prefilled package or cartridge. The gun has a first hollow chamber for retaining the package in fixed relation, the package containing the materials to be dispensed and either having a dispensing valve thereon or not as known in the art. A second hollow chamber of the gun retains an axially-movable hollow tubular piston for pneumatically forcing discharge from the gun and through a metering, mixing and/or dispensing nozzle. The hollow piston has a quick-release valve mounted internally in its head portion for rapid release of pressurized gas internally of the gun body for its relatively quiet operation. Internal release of the gas pressure within the head portion serves to prevent noisy operation of the gun. The gun is capable of economical manufacture with the valve components being formed from essentially non-machined moldable components.