This is one of the absolutely coolest prop ideas I've seen in a long, long time.
Raygun.es offers up a raygun construction kit of modular parts that fit together in an almost infinite variety of ways. The Spanish company is currently offering up 16 different resin parts ranging from bulbous bodies to stackable detail rings and Strickfaden-style adornments. It's an absolutely brilliant concept.
The mix and match construction style gives you a lot of flexibility in design. With the existing parts you can do everything from a Buck Rogers blaster to a retro-futuristic steampunk design.* Combine the castings with found parts and you can put together some truly unique props. They would also make ideal detailing pieces for non-weapon designs. Tillinghast Resonator, anyone?
*And to make it authentically "steampunk" you can sloppily glue on some brass watch gears in random locations! Epoxy blobs for the win!