According to the present invention, a gun is provided which has a valve assembly receiver and a product discharge valve assembly which are adapted to cooperate with each other and which may be readily and accurately secured together in a fast, fool proof manner. The product discharge valve assembly may be readily removed by hand without the use of tools, and misalignment between the inlet passages on the valve assembly and the outlet passages in the gun body are eliminated. The outlet and inlet ports respectively are aligned securely and positively by the combination of a V-grooved self-centering arrangement on the receiver and a counterpart arrangement on the discharge valve assembly. In a preferred embodiment further cooperating alignment is provided by a locking stud unit. The discharge valve assembly includes a reciprocable valve having a stem portion which may include a quick release lock arrangement for cooperation with an operating rod of the discharge gun, whereby the valve may be readily removed and replaced for maintenance in use.