Abstract: An automatic piston drive mechanism for use in dispensing flowable compound material through an orifice of a housing in which the material is contained. The housing may be that of a caulking gun or else that of a cartridge as insertable on a carriage of a caulking gun support frame. The drive mechanism comprises a piston member having an elongated rod and a pusher head. A drive motor is provided for imparting longitudinal displacement of the rod. A clutch member is rotatably driven by the drive motor and releasably connected to the rod to impart axial rotation thereto to effect axial displacement whereby the pusher head will apply pressure to the compound to push it through the dispensing orifice.