
Naturally it spurred on thoughts about pneumatic equivalents, here's a couple of thoughts. I would really love for a launcher that uses these to be semi-automatic, so apart from ease of manufacture, the critical performance elements are a fast gas dump to generate enough pressure to be able to recock the mechanism and load another cartridge through direct blowback, and a relatively weak force required to actuate firing in order to reduce the amount of blowback force needed for reliable operation.
Now that I have an efficient HP pump, pressures of a couple of thousand pounds are within the realm of possibility, increasing the potential blowback force but also the amount of force needed to actuate a valve, which requires some thought.

Coaxial exhaust valve somewhat simplified to exhaust through the "barrel". Since the spool is balanced and small diameter, even at high pressures it should need a relatively small effort to open. Filling can be done though the "barrel", either by connecting the mouth of the cartridge to a pressure source of by placing it in a pressurized chamber, similar to the fill rigs I had devised for my hybrid cartridge experiments.

Mechanically locked valve with a small difference between sealed diameters on the piston to limit the force needed to unlock it, similar in concept to the Mad Bull SS6 airsoft shotshell:

A proven design but definitely a little more complex to implement.