To cast multiple prop guns, PMC® 121-30 is an excellent mold rubber choice. The material is low shrinkage, easy to use (1:1 by volume mix ratio), and self de-airing. This application illustrates a cavity pour mold with a gypsum support shell. Ease Release® 200 is sprayed into the mold before the pigmented Smooth-Cast® 325 is cast.
In the picture below the mold is closed. Notice the legs used to hold the mold in a pouring position.
Here you can see the opened mold. One half of the rubber has been pigmented darker.