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Copper Semi-Auto

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Copper Semi-Auto

Unread postAuthor: skyjive » Mon Dec 08, 2008 4:04 pm

This is the first gun I've posted here, my contribution to the forum so far has mainly consisted of asking for help when I screw something up. I have made several guns before this one, but they were pretty basic single-shot guns, nothing that y'all haven't seen a thousand times already, so I didn't bother posting them.

This gun has 3/4" copper pipe as both barrel and chamber. The barrel is 53” and the chamber is 56”, not counting the cylinder and cylinder arm. There is 7.5” of dead space between the QEV and the feed gap (not good, I know). A schematic (pilot/fill system and chamber not included, top view):
Image

The gun works as follows: Air is fed into the rear of the gun from a 5 lb CO2 canister and regulator through a hydraulic hose via a quick-disconnect into the pilot/fill tee, and goes through the QEV into the chamber. When the gun reaches regulated pressure it stops filling. The cylinder support arm is made of ½” copper pipe and is integrated into the chamber, so that the pressure of the filled chamber pushes the piston out of the cylinder and closes the bolt. The bolt is made of ½” copper with the ends wrapped in duct tape to fit snugly into the ¾” pipe at either end of the bolt. The action of the bolt closing pushes the bottom round in the dropdown magazine into the barrel. The gun is fired by opening a blowgun, piloting the QEV. Chamber air is released into the barrel, firing the shot, and the loss of pressure allows the return spring on the piston to retract the bolt. The retraction of the bolt allows the next round to fall into place in front of the bolt in the feed gap, and the gun is ready to be repressurized.

As of now, all systems are go, and it has successfully autonomously fired multiple rounds in succession powered by a bike pump. The CO2 hookup which will give it a non-laughable rate of fire is still waiting on parts to arrive, but in theory it should work the same.

And now for some pictures:
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The entire gun

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The magazine (by far the most ghetto part of the gun, I basically made it out of a box lid and some duct tape, but it works great)

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The pilot/fill area, with blowgun pilot and quick-disconnect fitting

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Looking in the front of the magazine with three rounds loaded. They are sections of dowel wrapped in duct tape and are not intended to have any ballistic or terminal merit, just to make sure the feed system works. I plan on using saboted darts eventually, which is why I used a 3/4" barrel and have such a big feed gap and piston stroke.

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The sight picture when firing, looking through the magazine down the barrel.

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The business end.

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A top view of the pneumatic cylinder. Note the rubber bands to reinforce the return spring action.

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Bolt open, round chambered.

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Bolt closed, round loaded into barrel, piston extended by chamber pressure, ready to fire.
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Unread postAuthor: ALIHISGREAT » Mon Dec 08, 2008 4:08 pm

i like the tape LOL it gives it a ghetto look :D

i look foreward to seeing a video :wink:
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Unread postAuthor: iknowmy3tables » Mon Dec 08, 2008 4:18 pm

nice the idea of a bolt action actuated by cylinder has been mentioned before, but this is the first model I've seen made
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Unread postAuthor: Hotwired » Mon Dec 08, 2008 4:20 pm

Had me going what the fish there for a bit.

BUT ALL IS GOOD :)

The bolt closes forward and not backwards as I believe every single bolt action here has previously.

(insert copy/paste blather about dead space)

It's been a long time since I've seen a new design come along and to be honest I think it's brilliant.

Sure it can be improved but it looks sound, it's simple and it's worked.

If I was to snaffle your design the only thing I'd change is to have the bolt close backwards to lower the dead space between projectile and valve.

That could however have problems if the projectile was stuck as the air blast now wants to throw the bolt forwards and the cylinder at the front disagrees with it so your way is the safer move.

This could even be made automatic by having the cylinder operate a pilot valve at the end of it's travel after closing the bolt.

Great stuff :)
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Unread postAuthor: skyjive » Mon Dec 08, 2008 5:09 pm

Thanks for the comments! The cylinder-actuated bolt is obviously inspired by a couple threads I've seen on this forum, but the design is my own. The weaknesses of the design are the large amount of dead space required by having the bolt retract backwards into an empty area and the fact that I didn't really have any idea how to make the bolt itself except by taking the next pipe size down from the barrel and wrapping the ends in duct tape, which gives a poor seal and some flow restriction. Other than that, it works great, and the greater than 1:1 C:B ratio is intended to brute force its way past any inefficiencies. Originally it was going to be full auto with a pop-off valve piloting the QEV, but the pop-off didn't give enough pilot flow so I was forced to switch to a blowgun/semi-auto design.
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Unread postAuthor: starman » Mon Dec 08, 2008 6:13 pm

I applaud the general design and your inventiveness here. However, I feel you got too lazy on the construction and follow through...a rush job just to get something "working". If this a mock-up to find workability and flaws that's fine, but I would like to encourage you to replace the duct tape and cardboard with proper bits.

Yes, it does take lots of time and effort to do things right, but you'll be much happier when you do. As it stands now, this cannon will never stand the test of time.

Also, the direct links at Imageshack work better than routing us over there for every picture. Just click on "Show Adv Links" to reveal the direct link, then cut and paste it here and use the Img tags.
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Unread postAuthor: mark.f » Mon Dec 08, 2008 6:20 pm

There's nothing wrong with getting something done quickly, as long as you're willing to accept the outcome.

Good design, good implementation, overall an A+.
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Unread postAuthor: Copperboy » Sat Dec 27, 2008 4:41 pm

Wow, Skyjive! I too have heard and seen lots and lots of theories regarding the CHAMBER actuated bolt (never think the barrel dito will work as good in these low pressures (but of course I'd really enjoy if it did)), but I can't remember seeing one this functional. It's strange it haven't got more attention than this (yet ;) ) as it's really an achievement to get it working! Good jobb! :P

P.S A video would be awesome 8) D.S
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Unread postAuthor: btrettel » Sat Dec 27, 2008 5:43 pm

Nice job skyjive. As one of the few people who have made something similar I'm glad it's finally catching on. This is the second similar design I've seen and I like what I see.

Tape wrapped around something should actually work to make a sealing bolt... while it is ghetto, I think I'll try it.

My only real suggestion would be to eliminate the cardboard and duct tape. Polycarbonate sheet is fairly cheap when you buy cut-offs from a plastic or glass shop. Don't use acrylic--it's too brittle. PVC is if you have PVC pipe because you can cement the parts together if necessary. I have done that to construct special fittings before.

To respond to the people here who haven't seen a similar design before, yes, very similar designs have been done before. My recent FANG automatic Nerf gun uses the same cycle and works well. I've seen plenty of discussion and even some partially built guns that use similar principals. It's not new, but it is rare and it is a good idea.
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Last edited by btrettel on Sat Dec 27, 2008 11:22 pm, edited 2 times in total.

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Unread postAuthor: skyjive » Sat Dec 27, 2008 6:55 pm

Thanks for the comments guys. I make no claim to any originality on the concept, but making it actually work is more difficult than it seems. The tape seals work decently but lose a significant amount of air, I just don't have anything better to work with. There is no video because the gun performed poorly and I took it apart to use components for my blow-forward bolt semi-auto. The cylinder had cycling issues since the CO2 was being resupplied so fast that after a shot was fired the chamber was refilled before the bolt could fully return, so I had to hold the pilot open venting gas for a second or so to return the bolt fully. The gun worked, it just wasn't ideal. The blow-forward bolt gun is much more efficient, if a bit harder to construct.
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Unread postAuthor: POLAND_SPUD » Sat Dec 27, 2008 8:29 pm

hey it seems ok to me... sure it's ghetto but you can always fix it....
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Unread postAuthor: btrettel » Sat Dec 27, 2008 11:30 pm

Heh, I should check dates more often. I just realized this didn't come too long after my gun but I thought it was posted today!

If you're having trouble with the tape seals, I'd suggest telescoping brass tubes--they are available in most hobby stores in 1 foot lengths. They come in sizes that can all fit air tight into each other, so no machining to make parts fit is necessary. They can be hooked together into a longer barrel if desired too. I used these to make sure my bolt and barrel sealed. They'd probably work well in something like your blow-forward bolt design. I think I might try that.
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