One of the issues is that you need to depressurise the fill rig rapidly in order to shock the projectile into sealing (almost like an exhaust valve). I'm using a schrader valve to do this, and the fill rig is 26.5mm inner diameter, more than double the cartridge, so there's a lot of dead space.
I could scale up to 3/8" calibre and probably work better but the Sherline is a miniature lathe
One thing I discovered making this is (as LeMaudit had already pointed out, but you have to feel it to understand) is that it was significantly easier to cut the M10 thread for this one than the M12 for the pen gun, so making a small threaded burst disk holder shouldn't be a problem. It adds to the loading process but it should be more reliable.
Still, the "valveless" principle is too good to let go of without a fight. One thing I thought about was threading the outside of the cartridge in order to connect it directly to a schrader adapter, but it would be easier to just make a smaller fill rig directly.
Another option could be to have the projectile already placed in the seal, and with a separate fill valve in the back (that would also allow you to poke out any projectiles which failed for some reason...)
Yes, I have a lot of brass to go through
