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Semi Auto QDV Airgun

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 1:27 pm

I don't see it working, there seems to be no net force to push the bolt forward.

Is there some particular reason why you're going for needlessly complex designs that are highly impractical to build unless you have a machine shop or rapid prototyping machine?

What sort of power are you trying to achieve, with what projectiles, at what pressure, and to what size limits?
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Unread postAuthor: ByeBye » Mon Nov 29, 2010 1:36 pm

airgun pellets, .177 or .22, semi auto at atleast 500 fps, handheld
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Unread postAuthor: saefroch » Mon Nov 29, 2010 1:45 pm

I concur with JSR, this seems like it will be decidedly lacking in power. All your designs so far seem like they could be a full auto, but not a very effective semi. As soon as the valve begins to open, it will try to shut itself. A bolt that is pulled back instead of pushed forward is assisted by opening.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 1:53 pm

ByeBye wrote:airgun pellets, .177 or .22, semi auto at atleast 500 fps, handheld


Define handheld, would something like this be acceptable?

Image

Unless you have a high pressure source, achieving that sort of performance with a multishot means it's going to be pretty big, even with the best possible valve.

With 100 psi and the equivalent of a burst disk valve I never got more than 400fps with 0.177" lead pellets in a 30" barrel.
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Unread postAuthor: ByeBye » Mon Nov 29, 2010 1:58 pm

Why is it lacking power?
The orange part is one piece btw.
So I have to make it stay in place when firing?

Define handheld

Tiny bit smaller :P, 4 feet?
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 2:19 pm

What's the objection to making a relatively simple piston valve like so?
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Unread postAuthor: saefroch » Mon Nov 29, 2010 2:52 pm

ByeBye wrote:Why is it lacking power?
See my design on the previous page (just as an example). The piston is currently slid forward, in the sealed position, filling. The operator uses the rod in the back to pull back on the piston. As soon as the the ports to the barrel align, the valve unbalances, just like yours. The only difference is that my design (like a QDV) takes advantage of the unbalancing. Pushing the bolt forward requires the operator to push against the piston to open the valve and also to prevent it from shutting again (Hint: this might make for a good full-auto, as JSR said). Why? When the piston is fueled, there is an equal area and equal pressure, thus equal force on both sides of the inside of the piston. Once it begins to vent out the barrel it acts like a blowback bolt and tries to throw itself backwards because of the pressure differential. The area subjected to pressure of the gas on the back of the piston has not changed, but the area on the front has gotten smaller. Thus a net force backwards.

ByeBye wrote:So I have to make it stay in place when firing?
I'm not sure what you mean by this...
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Unread postAuthor: ByeBye » Mon Nov 29, 2010 3:34 pm

saefroch wrote:See my design on the previous page

I don't understand it, looks like the pressure inside it would push it to the right as soon the pressure would increase and stop the refilling.
And the reloading mechanism is missing.

I think this is what you mean? (see attachment)

saefroch wrote:I'm not sure what you mean by this...

nvm
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 3:46 pm

What pressure source are you planning on using for this?
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