SpudFiles

Well after a spree of pen gun's and JSR's latest full auto works, I came up with this idea. It was inspired by the piston used in his gun and finding some thick alum. alloy pipe that a syringe sleeved perfectly in.

It has some design flaws at the moment, these being:

-Multiple holes in chamber.

-Tap and Die sets required for threaded barrel plug/spark plug. (I have a tap and die, speaking in general...)

-Schrader valves limit the mix capabilities.

-Epoxy plug for the rear schrader, should hold but would be risky without substantial tests. In theory it would hold as it only contains the pressure between it and the syringe, with a bit more as the piston is forced back during combustion.

-Limited mix capacity to offer safety margin for handheld firing.

-Use of stronger materials required.

-Difficulty in building the barrel plug. It may have to be permanent, making purging difficult.

Any thoughts/suggestions/weigh ins?

Well, it follows the same design, albeit with a coaxial layout, as other piston hybrids that work, so it should too.

To address the problem of weakening the chamber wall by having the two holes in it, perhaps you could fill via the barrel. Thread the muzzle or possibly a quick connect larger than the barrel ID. Then set the piston pressure to your fuel pressure and fill. Reset piston pressure to air pressure and fill. Reset piston pressure to desired pressure higher than mix pressure.

Or if you have an external meter, just have the piston pressure at zero, then raise it to final pressure before disconnecting the fueling line.

I would also definitely look at reinforcing that epoxy plug with screws, especially with the force of the piston coming back on it. If you made it with o-rings, and not actually fixed to the pipe, but held with screws behind it, you could remove it to adjust the spark gap or replace the piston.

I dont feel much for threading the schrader and spark gap into the chamber wall like that. Maybe use T-pieces (or one cross) for that.
How do you plan to meter fuel?

How about an extended version of this hybrid cartridge idea?

Not very practical way of sealing the projectile for lots of shots though.

you could fill though the barrel by having a low PSI bumper behind the piston ... then increase the pressure behind the piston after fuelling... this would however be inconsistent...