The inner gun was the .50 cal Browning. When not fitted the "domed" cap was used to blank-off the blast tube.
If the .50 cal was fitted, then the "tube" was blunt, as depicted by the kit part, and sealed with a dull red fabric patch, inside the recess as moulded on the kit parts
I can only assume that the kit parts allow the option of having a "sealed" stub tube, or, by hollowing this, and fitting the part for the gun muzzle, being able to show the stub with the muzzle visible, as if either not yet sealed, or "shot through" after use.
Obviously, not having the kit or instructions, I can't positively confirm this, but I'm guessing that the gun muzzle parts can be left off. just fitting the stub housings.