my idea would be to make half of the gun, say make one side, and make the other, and make it so you just need to connect a few tabs and the two become one piece, resin and glass the two halfs inside, then glue to tabs and make the gun 1 pep piece, then resin the outside.
Btw, NEVER USE PRINTER PAPER FOR PEPAKURA OR PAPERCRAFT
it will sog down and die.
Cardstock never sags down when resined unless it's overweighed