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Ring Piston

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Ring Piston

Unread postAuthor: Secret Squirrel » Sat Oct 23, 2010 11:14 pm

What originally got me into spudding was building a launcher for airsoft. Originally I wanted to build an AT-4, but unable to find a cheap spent tube, I decided to go with the Carl Gustav M2 instead.

My first attempt was a coax with a 1.5" barrel and 3" chamber, which worked surprisingly well for my first cannon, however I never liked that it had to be muzzle loaded while the real Carl Gustav is breech loaded.

I think I've come up with a solution for breech loading a coax. Instead of having the piston seat on the end of the barrel, it instead is ring shaped and fits between the barrel and chamber. It seats against a modified coupling around the barrel to create the necessary difference in surface area for the valve to work. When the valve is piloted, the piston slides back uncovering QDV style ports in the barrel.

This lets the end of the barrel extend out the back of the launcher, allowing breech loading through a threaded plug or full bore ball valve. Works in theory, though I see a few drawbacks, namely the large ratio piston.

I'll build a prototype if I get a chance, somewhat busy right now with college.

[edit] after a few minutes more thought, I realized that the pilot would leak into the barrel unless there was some sort of O-ring. So I guess this is going to the "nearly good idea pile." I'll keep trying to make the idea work, but it looks like it will be a bit more complex than originally planned.
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Unread postAuthor: lozz08 » Sun Oct 24, 2010 2:24 am

I don't know if anyone has ever come up with a coax breech loader. You'd probably go insane trying to put it together.
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Unread postAuthor: POLAND_SPUD » Sun Oct 24, 2010 7:06 am

with a coax breech loader
doable... but you need a detent instead of piston valve

though you can build a piston valve gun exactly as you described - but it has to be a combustion/hybrid. Then you wouldn't have to pilot it - a spring behind the piston would be enough
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Unread postAuthor: Technician1002 » Sun Oct 24, 2010 9:43 am

A QDV built into a breech bolt would work. Breech loading through the valve and closing the bolt would assemble the valve. I played with the design, but never built one due to limited machine shop access to build the breech bolt.

It would add considerable length to the breech of the cannon.
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Unread postAuthor: Secret Squirrel » Sun Oct 24, 2010 11:25 am

lozz, I've been going insane for a couple months just trying to figure out how to put it together.

Poland, I already tried the combustion idea, but after some testing I decided it was a bit dangerous for airsoft use, it burnt a few of the projectiles I tried.

Tech, I had thought of something similar, but I didn't like the idea of having to remove the piston every time, I needed to reload.

I think I may have come up with a solution to the pilot leak. In my first coax I used a bit of shop vac hose as a bumper. If I connected the piston to the back of the pilot via a collapsible hose around the barrel it would seal off that leak, and perhaps make an effective bumper, though it would make it practically impossible to remove the piston for servicing...

I'll keep thinking about it.
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