Brazed 12 gauge update + Potential Jig idea for quick small scale production
So after some careful consideration I think instead of the original Nylon rod / 3d printed base plug idea i'm better off using a length of 18mm threaded rod and just threading the inside of the shell then screwing in and brazing the plug in place. Doing so makes an entirely metallic and very strong cartridge, I'd argue the shell casing has a better chance of failing than the cartridge base with this design.
I've also designed a brazing jig, the plates are ceramic tiles with holes cut through them to keep the washer and tube perfectly cantered with one another, the top tile rests on top of 4 nuts threaded onto the rods. A small clay disc is pressed into the tile at the bottom to keep the top of the washer roughly level with the top of the tile to make brazing easier. When its done you just pull the top tile off, remove the shell and boom. Its a solid piece.
Also I'm not too sure if I can even call this 12 gauge anymore because technically speaking the cartridge is roughly 12 gauge (20mm) but the bore of the cartridge is only 18mm due to the wall thickness being 1mm, meaning if you made an 18mm slug for this and loaded it in a conventional shotgun its probably going to tumble around the barrel, have near zero pressure behind the bullet and probably damage your gun. I imagine something like buckshot/birdshot will probably be ok but not as good as an 18mm barrel with a 20mm chamber specifically designed for this round.
So I guess i need a name for it, 18mm BSS? Brazed shot shell.
I'll come up with a set standard of dimensions and tolerances if enough people are interested in making this a standardized round for DIY applications.