Hey dude, you have a cool idea nonetheless. May be more useful than a Judge (can't see how it wouldn't, but I haven't seen your design). Anyway, if you do indeed have a good drawing, copy that and do a writeup on it and how it would be use and maybe include some persuasive paragraph to sway them your way and send it off to Technology Branch. They can at least look it over and give you some advice. Ask for an exemption. Some weapons you'd think are DD aren't. The Serbu Super Shorty, it is an AOW (perhaps for other reasons, but still...) and isn't that 20mm sniper rifle exempt too? You gotta get a letter, but they can at least tell you I'd think based on a good set of drawings, diagrams, and information and writing.
If you have the shop to buid it and the material and know how, then this may save you some grief. Worth checking out. Because of the size, weight and limited capacity of a 12ga. revolver, along with the fact it will be single or small production, they may be okay with it.