For the helmet I'd say to do a 2 piece mold. Lay it on a bed of clay (or just put a line of it around the bottom edge) right side up and pour the silicone over it in a box. Once that cures, take it off the clay and flip it upside down. Brush or spray on some kind of release (Vaseline works well if you don't have a spray release) on the bottom of the silicone to form a thin "barrier" on top of the cured silicone and pour another layer of new silicone inside the helmet and over the cured silicone. Be sure to have the release agent in a good thin layer or the two halves of silicone will bond and you'll need to cut them to demold the helmet. You might want to do some "registry keys" so that the two parts fit back together exactly how you made them. If you do a search for "two piece mold" you'll see how some have made the keys. The gun can be done the same way but make sure you get the release agent in the trigger area or you'll have to cut that to demold it. Hope this helps.