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Simple yet ghetto semi-auto design

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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Simple yet ghetto semi-auto design

Unread postAuthor: mister pointy » Thu Jun 19, 2008 10:12 pm

Hello everyone.
I am new to this forum but i have some experience with spud guns.
This is my third gun, but my first pneumatic.

some specs: 3'' chamber, about 12'' long
2'' secondary chamber, about 13'' long
the barrel in the pictures is 2'', 2' long

This gun has evolved over time as i have modified it as i go along. It started out as a simple in-line with an electronically activated sprinkler valve. I then modified the valve to be pneumatically actuated, and chopped my barrel to make it an over-under configuration, and retrofitted it for use with the barrels from my old combustion gun. I was reasonably satisfied with the increase in power i achieved with a simple modification, but as time went on i wanted more. I toyed with the idea of making the gun semi-automatic, but had no idea how to do so.

I saw a design on this forum which used a modified blowgun to make a three way valve, effectively firing the gun and reseating the valve in one pull of the trigger, but it required modifying the blowgun.

I decided that i wanted to make an easy semi-auto setup for a sprinkler valve that would not only be simple, but also require no parts to be modified, so that it would be easily replicated by anyone without acess to expensive tools.


the setup is simple: a 1/4'' tee is attached to the sprinkler valve where the blowgun would normally be, the blowgun is mounted on one side of the tee, and on the other side is a ball valve followed by a 1/4'' hose connected to a secondary air chamber. The ball valve is opened slightly to let air from the second chamber through.

The idea is that the ball valve lets the air from the second chamber into the valve fast enough to reseat it, but not too fast that it all vents through the pilot valve (the blowgun).
Its not perfect at all, it takes a lot of fiddling to get the flow correct. if the ball valve is open too much it lets too much air out of the pilot and the sprinkler valve does not fire well, it doesn't let much air out. On the other hand, if the ball valve is not open enough, it takes longer for the valve to reseat, and air gets wasted. I get about 3 or 4 good shots out of it at 60 to 80 psi. my compressor is horribly small and it doesn't get my chamber all the way to 100 psi.

I know this design is not great but it is a cheap and simple way to get a couple more shots out of a gun with a larger chamber.

If anyone has any thoughts or comments they are welcome.
My main objective with this gun was to create a simple way to get more shots out of a large chamber.
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Attachments
airgun1.JPG
the whole thing
airgun2.JPG
view of the valve setup
airgun3.JPG
other side, with huge primer stain, don't worry, i painted it.
airgun5.JPG
business end, that crappy inner tube valve was removed and replaced with a threaded schrader.
airgun6.JPG
size comparison, that is a spray paint can
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Unread postAuthor: THUNDERLORD » Thu Jun 19, 2008 10:41 pm

Good job there.
I am wondering if you could get the ball valve set to just the right spot and then position it there permanently?

Good idea, I have a few sprinkler valves I'd like to use for testing something.
Thanks 8)
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Unread postAuthor: daxspudder » Thu Jun 19, 2008 10:58 pm

Very nice build, I like to see a new design. Have you thought about having a breach loader? That way you can fire quickly, rather than just getting several shots on one charge that take considerable reload times in between shots... great idea, but I'm sure that there is something better to use than the ball valve.
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Unread postAuthor: Pilgrimman » Thu Jun 19, 2008 11:12 pm

Instead of a ball valve, you could just connect the hose directly, and alter flow by tightening or loosening a hoseclamp around a section of the hose. I've never done it, but it's worth a try, and once you get it right, it's permanent!
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Unread postAuthor: mister pointy » Thu Jun 19, 2008 11:49 pm

Thanks for the suggestions,

As for permanently affixing the ball valve:
The valve setup performed differently than i thought it would at first. As the launcher is fired, the air pressure in the main chamber drops, therefore the pressure in the secondary chamber needs to be regulated so that the valve will fire. it's a bit tricky to explain, but in order to keep the air from just leaking out of the pilot, the valve needs to be adjusted a little each time the gun fires.

I actually do have a breach loading barrel, it's just a real pain to put on this gun cause i have to take it apart to get it on there, i just left the 2'' barrel on there for the pics.
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Unread postAuthor: Spud King » Wed Jun 25, 2008 8:32 am

Some constructive criticism:
You need to take the sprinkler valve off and re mount it just after the last elbow, before the barrel. This way there is much less wasted air space that needs to be filled by each shot.

Neat idea, good job, it reminds me of this:
http://www.members.aol.com/markfh11q/fullauto.html

I wonder about both of these projects: closing the sprinkler valve while it is trying to open probably cuts performance a lot.

A better way to achieve the same thing would be to put that barrel and sprinkler valve onto the smaller chamber and link the chambers together with a small piece of tubing. And just have the sprinkler valve operate normally. Then when it shoots, it shoots all the air in the smaller chamber (and both elbows) as affectively as it can, and then the larger chamber fills the smaller one for another shot. I just went out and tried it on my sprinkler valve gun, (to make sure I was right). I left my tank attached to the air compressor and fired the valve, then let go of the trigger, and the chamber refilled for another shot at the same time the valve closed. Then I shot it again, and again and… (The ‘trigger’ on my valve is a pop off over pressurization valve that only pops off when I pull it with my finger and when I let go it closes.(It has a ball valve safety just after it because it can be pretty sensitive )) For such a gun I would recommend a backpack of 2L pop bottles pumped to 100psi (with plywood between you and it) instead of the larger chamber as an air source. (which seems to be too small even for the 2” barrel you have right now.)

Ps
Pilgrimman that’s a great idea, I would not have thought of it.
:)
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