Oh I finally understand the question now.
If you want to screw the two halves together, you must make the mold with a screw in it in one of the halves, or go to the home depot and buy a screw in piece, it's a small piece which you can epoxy or glue into plastic and it has the input for you to screw a certain sized screw into.
Molding the screw along with the piece may also work, then on the other half, you just drill a hole take out the scre, connect the two plastic pieces and screw them in, but then the screw not supposed to be there adds inaccurate detail. Hot glueing/bondo/epoxying guns seems the way to go, otherwise, you could carve the weapon from clay, plaster mold it, then make a solid resin cast of the inside....or hot glue fill the plaster mold, then th entire gun will be one piece.