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another semi-auto idea!?

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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Unread postAuthor: ilovefire » Thu Jan 27, 2011 3:56 am

thank you, here how about i show you, yes the bolt is still a little bit broken but it is still functional
sorry about the feet :?


[youtube]http://www.youtube.com/watch?v=iC9l3QoIK8s[/youtube]
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why make it if it dosent shoot?
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 27, 2011 4:19 am

lol the first thing I did was cringe at the feet!

Yes, it seems we got the same results (and clide had reported similar "angry wasp" performance with his BFB) - there's no way you're going to reliably feed projectiles and get decent power at that rate of fire. It's perfect for a semi-auto though of course, where the valve is delivering a pulse of air and not constant flow.

In order to make it work in the latter case you need to introduce a "pop-off" element, as discussed here.

Image

I included a cut-off valve in the above animation but in practice the design should funtion well without it.

There are other ways you could combine a pop-off piston with a reloading bolt as shown below, but combining it with a blow forward bolt as above gives you a more elegant "straight through" design.

Image
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Unread postAuthor: ilovefire » Thu Jan 27, 2011 6:04 am

im not sure if i just dont get it properly but would the first gif do the same "angry wasp" affect? and with the bottom one wouldnt that have issues with pressure pushing the bbs buck up the mag? sorry im not trying to do away with all your ideas in fact the last one did help i think it gave me an idea ill post it as soon as i can draw it up, thanks
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 27, 2011 6:51 am

ilovefire wrote:with the bottom one wouldnt that have issues with pressure pushing the bbs buck up the mag?


It's assumed that by the time the bolt reaches the mag opening it's assumed that most of the air would have gove out the barrel. Still it's advisable to have a spring fed and sealed magazine.

ilovefire wrote:im not sure if i just dont get it properly but would the first gif do the same "angry wasp" affect?


The pop-off element (piston>piston seat) should introduce a delay which brings cycling down to a more realistic rate. It's important that the bolt piston is smaller in diameter to allow air to leak past, but not enough that the difference in area between bolt and cylinder is bigger than the area of the piston seat.

For example, with a piston seat diameter of 6mm the area is 0.282 cm<sup>2</sup>. Say you have a bolt housing 12mm in diameter, that's an area of 1.131 cm<sup>2</sup>. A bolt piston diameter of 11mm gives an area of 0.950 cm<sup>2</sup>, so the area through which air can flow past the piston is 0.181 cm<sup>2</sup>, smaller than the seat so an overpressure between the seat and the piston face, and it can pop effectively.

The bolt might not move forward enough to seal the magazine port, so it's still worth having a sealed off mag, but it will definitely block it off and avoid double feeds.

I feel a prototype coming on...
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Unread postAuthor: ilovefire » Thu Jan 27, 2011 7:38 am

are i see how it works now, makes sense


jackssmirkingrevenge wrote:I feel a prototype coming on...



ah, so iv inspired you :wink:

anyways this is what i was thinking, earlier today i was using my pen gun to see if it worked and after seeing that other gif it somehow made me think of this, might even do it as a coax but im going think about it and see what i could get more compact
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 27, 2011 7:46 am

Had you seen my semi-auto, it had a similar valve configuration piloted by a schrader but with a different planned layout:

Image

I still think that a blow-forward bolt on a semi-auto firing spherical projectiles is a needless complication though, a simple tee magazine will work just as well assuming it has an airtight magazine with the part closest to the barrel the same inner diameter as the projectile.
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Unread postAuthor: ilovefire » Thu Jan 27, 2011 8:01 am

yea i know i do tend to over complicate things a little, but for some reason im hell bent on having it with a horizontal mag, kinda like how you wanted the flying brass, also probably because it would then have the option to have about 50 bbs and the way i want to be able to realode it wont be anywhere near air tight, plus iv already got the blow forward built so all it would require is for me to make the piston valve and there not that hard, just a quick question, what would be the best way to have the piston valve semi-auto, fill behind the piston with a pinhole or from chamber side with a spring behind the piston? not necessarily the easier way, whatever is more air efficient?
thanks
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 27, 2011 8:10 am

ilovefire wrote:for some reason im hell bent on having it with a horizontal mag, kinda like how you wanted the flying brass, also probably because it would then have the option to have about 50 bbs and the way i want to be able to realode it wont be anywhere near air tight, plus iv already got the blow forward built so all it would require is for me to make the piston valve and there not that hard


Fair comments :)

fill behind the piston with a pinhole or from chamber side with a spring behind the piston? not necessarily the easier way, whatever is more air efficient?


From what I've seen option 2 would be most efficient.
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Unread postAuthor: ilovefire » Thu Jan 27, 2011 8:20 am

ok thanks, ill have to look at the stuff on my desk and ill probly put something together over the next week, hopefully
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Unread postAuthor: ilovefire » Fri Jan 28, 2011 2:48 am

ok well i did up a quick prototype coax with a schrader in the chamber and spring behind the piston will be testing it for semi-auto later this afternoon, lets just hope it works, anyways heres a pic
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