A filling head gun assembly has a housing; a handle assembly connected to the housing; a filling head attached to the housing; and a plunger. The plunger extends through an opening of the housing and an activator reservoir attached to the plunger. The reservoir is moved into position to contact the filling head via the plunger to feed pressurized activator through the filling head to an aerosol can. A cover is releasably attached to an end of the filling head which provides a seal around the filling head.