Here's a semi-auto combustion design...

[auxiliary]

So this is how it works,
When you pull on the trigger, it is attached to the Delrin hop-up/feeding system so it and the barrel moves back. As it moves back the BB in the barrel is pushed forward until it is between the two o-rings, by the small brass tube mounted on the 1/4in aluminum plate. By the time the BB is between the two o-rings, the back of the trigger will have depressed the push button igniter causing a spark in the chamber.

When the combustion occurs, it will shoot the BB out of the barrel.
Then the hollow metal piston will be pushed back past a small hole that lets in low pressure propane. The piston continues back until is reaches the large vent holes in the copper pipe. Hopefully it will continue to back a bit because of its momentum and weakness of the spring. The exhausted gases blow out the vent holes and as the gases cool, it vacuums in new air from the atmosphere. Once the piston moves forward sealing the vent holes and passing the propane hole (which has been letting in a little bit of gas hopefully the right amount depending on regulation), the gun is armed.

[littlebro05]

Wow, makes me want to build one, btw auxiliery did you draw that up with microsoft paint? cause the file was bitmap. Cause if you did it's very beautfiul, and quite easy to understand.

except for the piston part, I'm assuming it's abit off scale the drawning?

[auxiliary]

Thanks for the complements! The piston is the size it is supposed to be and everything is almost perfectlt to scale (100pixels = 1 in.)

Are there criticisms about my design or is it absolutely perfect (which I doubt).

[Ragnarok]

It's nice, but my main concern is that the piston is unlikely to recoil backwards, and ensure all the combusted gases in the chamber are cleared.

Also, that design will spew propane to atmosphere as the piston recoils. You really need to vent, then fuel.

However, with a good chamber fan, you might sort the first problem, and the second is clearly not insoluble.

[auxiliary]

If I somehow vented and then fueled using a different sort of mechanism, would the venting idea that I have currently of a piston going past holes in the chamber be sufficient? Also are you basically saying that the combustion in the chamber will be so quick and not provide enough force to move the piston back even if the spring is really weak?

[Ragnarok]

If I somehow vented and then fueled using a different sort of mechanism, would the venting idea that I have currently of a piston going past holes in the chamber be sufficient?

Perhaps, it will depends.

Also are you basically saying that the combustion in the chamber will be so quick and not provide enough force to move the piston back even if the spring is really weak?

Sorry, mistype, it should have said:
"the piston is unlikely to recoil backwards far enough, and allow enough air flow to ensure all the combusted gases in the chamber are cleared."

Sorry, posting fail.

[auxiliary]

Oh okay, because I was worried that I wouldn't be able to utilize a blow back mechanism. You can expect to see more designs from me in the future.

[Antonio]

hey nice design! Btw which program did you use to make this diagram?

[ramses]

I could see a bolt handle on the bolt, as well. the propane, also, has no metering designed into it as far as i can tell. I would suggest a spool valve, as to solve this problem. It will also keep it from "spewing propane to atmosphere." good luck, and which program did you use for the rendering? It looks great.

[psycix]

Maybe use pressurized air to blow fresh oxygen into the chamber, at the same time that the propane gets in.
Getting the right amount of fuel in would be a little tricky, however.