[Gandalf_The_Gray]

Shove a pellet into the front end of a cylinder, meter acetylene into the center of that cylinder at 10 psi, then abruptly slam a piston into the back end...

Sounds promising, you'd probably need cast iron piston rings to contain the gas pressure and a spring mechanism like the single shot spring/piston air guns at least for "proof of concept". It might be possible to let the spring piston rebound, cocking it for another shot. Full auto may even be possible. The characteristics of acetylene are such that you don't need air or oxygen if you can get it to detonate reliably from heat of compression it makes for a much simpler mechanism.

I remember a long time back the old Science and Mechanics magazine had a cover story about a rocket powered go cart. The exhaust gas from a single cylinder two piston engine was ported through a nozzle along with steam from the engine cooling jacket. The two pistons were not connected to any output mechanism. They ran a two cycle porting with bounce chambers to return them to top dead center. It ran on acetylene and reached something like 20,000 cycles per second. The inventor called it "loud mouth". Hmmm

Regards,
GtG

[null and void]

Yes. I’m thinking a structural plastic frame and piston with a steel spring, brass (or steel?) tube sleeve and cap on the piston.

[Gandalf_The_Gray]

It was really late (early?) when I wrote this so:
"20,000 cycles per second" should be "20,000 cycles per minute"

Regards,
GtG