It would be great to see multiple cartridges in a gun, good luck with your progress.
Thanks, as it is there is a lot of promise in that even without the benefit of a breech to contain the expanding gasses, the thing still happily throws itself across the room indicating it should be capable of operating a direct blowback mechanism, countering the force of the bolt spring as well as the friction caused by the following cartridge pressing against it.
It's a shame that I can only use it at 6x when I know that piezo ignition allows me more than twice that, but freedom from the weight and complexity of a burst disk outweighs this disadvantage. Also, the great thing about cartridges is that it makes the design modular, so in future I might yet find a better sealing material or construction technique that will let me up the pop-off pressure.
Heating was a big problem for the barrels, mine warped from using thin walled tube.
Heat is a particular concern as the barrel I'll be using is cast from epoxy and the breech is a section of 3/4" PVC pipe, so there's no metal in this area at all. The consolation is that most of the heat is absorbed by the cartridge body which is ejected so this shouldn't lead to problems, especially as this is going to be a semi-auto launcher.
Currently curing is a 1.25" long breech attached to a 6" barrel, HGDT predicts a fairly low 250 feet per second for a 3/8" bearing which isn't exactly spectacular but I think it will be enough to inspire me to actually make a working cartridge fed prototype.
More testing planned for when it cures, as always watch this space