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Could Semi-auto be this easy?

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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Could Semi-auto be this easy?

Unread postAuthor: jor2daje » Mon Sep 07, 2009 2:02 am

I came up with this rough design that uses a three way valve to pilot and fill a qev. But instead of using a blowforward bolt or a t-mag I plan on linking to bolt directly with the trigger. Because this is only airsoft calibre I only need the bolt to move 6mm before the 3 way valve is triggered.



Any input or improvement on this design would be appreciated.

(I dont know how to upload the sketchup file itself, but if anyone wants it I guess I could email to them.)
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How it should hopefully work.
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Unread postAuthor: inonickname » Mon Sep 07, 2009 3:38 am

Yes. A similar setup has been used before. Hell, you can even set up a pneumatic ram using this setup to operate a breech for aclosed-bolt weapon as well.

Edit: You'd want something with more flow than the restricted head from a blowtorch and something to stop liquid propane entering the chamber.
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Unread postAuthor: btrettel » Mon Sep 07, 2009 8:09 am

A number of people here (myself included) have done essentially the same thing. In my gun I considered linking something to the trigger movement during different parts of the design phase; none survived to be built, primarily because too much draw was required. For an airsoft BB however, typical trigger draws are more than adequate.

From what I'm seeing in your approach, you're going to use telescoping tubes as a breech and barrel. That should work.
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Unread postAuthor: POLAND_SPUD » Mon Sep 07, 2009 8:38 am

yeah you might give it try why not... of course it would be easier to implement with a slide valve...

inonickname is right that you might as well use an air cylinder to do the job (since you've already got a 3 way valve)... also I think it would be easier to build this way



in your pic the 3way valve will experience a lot of stress... normally, using threaded fittings is a nice way to attach everything together and guns built in this way are really tough

but 1/8" is too small IMO and because normally you would be holding the pistol grip the mass of the tank itself would put a lot of stress on the 3 way valve (and the fact that its body is made from zinc doesn't help)
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Unread postAuthor: jor2daje » Mon Sep 07, 2009 12:14 pm

Your right poland spud, I think Ill put in some frame work in to support everything, this is more an idea of the internals.

The pneumatic cylinder idea is good but I only have one cylinder so I am planning on using that on a semi auto gun that uses non-spherical ammo. So I can have a breach that moves about an inch with each shot.

inonickname, do you think i could bore out the opening on the torch head to make more flow, even if not I dont think it will take long at all to fill such a small firing chamber.
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Unread postAuthor: POLAND_SPUD » Mon Sep 07, 2009 2:57 pm

ohhh right... you've bought 3 valves :D

did you consider using unregulated propane ?? that would make things easier...

and it would be a good idea to attach a length of malleable iron pipe and/or fittings to the tank...


first of all, it could be used as a framework to attach the other parts (e.g. by joining it with the chamber, of course I am talking only about mechanical joint - you'd have to block off both of them but that's easy)

secondly, that would mean extra surface that will absorb heat from surroundings and, as a result, reduce pressure drop (that's true both of regged and unregged version)

ohh BTW.. you've made a very neat sketchup model... :)
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Unread postAuthor: jor2daje » Mon Sep 07, 2009 6:23 pm

Ya poland, the propane is gonna be unregulated Im just using a torch head to turn it on and off.

I was thinking of using abs or pvc to make up a framework, with some pipe to hold the propane tank and other fittings encasing and supporting other things. I could use some bondo and hopefully make a fairly nice looking gun.

I do like the idea of using extra area around the tank to regulate temp/pressure thats a good idea.

And thanks :D (although i mainly just took deweys parts and assembled them)
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Unread postAuthor: POLAND_SPUD » Mon Sep 07, 2009 8:06 pm

well I am not an expert when it comes to PVC but it seems that it would be too weak to support the tank...

I've uploaded a pic.... notice that it doesn't differ much from your design but offers a very rigid structure...

IDK really what thread type is used on the that type of tanks... but I am sure there are adapters to NPT threads
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Unread postAuthor: jor2daje » Mon Sep 07, 2009 8:16 pm

Thats a pretty cool design but, I also came up with redesign, Im gonna get rid of the elbow in the firing chamber, and integrate a grip over it. This should make the design a little less awkward.

I'm not sure either but I think a piece of pipe that fits fully over the propane tank should easily support it aslong as I stay with large diameter pipe.

If they make an adapter from propane to npt, I've never seen it, I just take the internals out of torch head and solder whatever fitting i need onto it.

(i'll change my sketchup design in a little and how the redesign looks)
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Just a quick adjustment, hopefully cleans up the whole look, adds support to the handle, and free up the top of the barrel for proper sights.
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Unread postAuthor: POLAND_SPUD » Mon Sep 07, 2009 8:53 pm

I though about integrating the grip too...

sleeving the pipe over the whole tank seems like a good idea... but still you have to attach the pipe to the rest of the gun...
of course all the parts of the trigger&bolt mechanism would be hidded, which is probably the most important thing on this gun

there are NPT to propane threads adapters... however, since those tanks are quite rare in europe IDK exactly what threads they use... but I am sure that someone here knows that
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Unread postAuthor: jor2daje » Mon Sep 07, 2009 8:58 pm

Ya the more I think about it I like your galvanised steel idea, I have some steel and I think ill just weld up a frame and bolt the valves and stuff to that, it may be sorta heavy but shouldnt be too bad.

I'm sure the adapters exist but, Ive already soldered mine up, basically the internals of this gun are all ready and I'm just waiting on the 3-way valves to get here (hopefully tomorrow).
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