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Shoulder-Mounted Coaxial Concept

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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Shoulder-Mounted Coaxial Concept

Unread postAuthor: surfbum » Thu Apr 19, 2007 11:48 pm

hey guys
just wanted some input on design concept I'm working on
and let me start by apologizing for the crappy diagram

some background info:
1. this is meant to be carried around with the ability to fire quickly and shouldered like a bazooka
2. the overall length is going to be approximately 40", half of that is the barrel.
3. the reason this does not look like a standard coaxial is because the barrel will be able to lift up, enabling breach loading.
4. the barrel will be 2" ID, as will the chamber. the "power tube" which supplies the air from the chamber, to the barrel will be 1/2" ID
5. the entire cannon will be constructed out of aluminum with the exception of the piston (which will be made out of delrin)
6. the cannon will run off a small regulated CO2 canister at about 100-125 psi


a question I have:
In the close-up design of the piston (I know its kind of hard to see) I've hollowed out the inside of the piston to make a check valve. my question is, how much better performance will I see with the check valve as opposed to just a small equalization hole? It is not hard to make but is it worth it?

if you have any questions/comments/suggestions I would love to hear them
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Unread postAuthor: hi » Thu Apr 19, 2007 11:55 pm

you really dont need an equilization hole or a check valve. i have built 4 piston valves and none of them have check valves or euilization holes or O rings. all you need to do is make it fit well and the gun will do thee rest. :wink:
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Unread postAuthor: mega_swordman » Fri Apr 20, 2007 5:17 am

My main question is, why would a shoulder-mounted coaxial be different from a normal one? In my eyes, it is simply the way you hold it or where you position the handles.

Other than that it looks fine, though my main concern would be hooking up the small pipe in the center to the barrel twice the size.
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Unread postAuthor: goathunter » Fri Apr 20, 2007 11:58 am

Considering the time it would take to build a small check valve and then the added weight.Stick to the tried and true method of the equalization hole.But,go ahead and try the check valve.If it works out better I'm sure some of us will try it out.
BTW:When you are finished with the gun post the plans and pics.I'd be interested in building a MKX look alike.Thanks
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Unread postAuthor: scatdawg » Fri Apr 20, 2007 12:01 pm

the only concern with the check valve is added weight. but if your pston design is light weight before the check valve it may not hurt performance.


edit. i just built a cannon almost just like this and i put two 1/64 holes in my plastic piston under the rubber flap works great. i dont know if a check valve would work any better. imho i think it would just be some work for nothing.
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Unread postAuthor: clide » Fri Apr 20, 2007 12:10 pm

IMO a check valve is worth it, especially if you want to use a small exhaust valve. I put them in almost all my piston valves.

Instead of having the "power tube" you should look into having a Toolies style setup seen at http://my.freeway.net/~toolies/ . This will allow you to increase the size of the outlet without reducing chamber volume as well as eliminate the performance decrease of the long tube before the projectile. You should be able to make the outlet about 1.4" with that design. That's almost 8 times the flow of a .5" outlet.
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Unread postAuthor: goathunter » Fri Apr 20, 2007 3:27 pm

Clide,The gun has to be coaxial to be a MKX look alike.How did you build the check valve in the piston?And is the checkvalve better in larger pistons or just the smaller?
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Unread postAuthor: lukemc » Fri Apr 20, 2007 3:34 pm

why not just add a T joint in the middle ot the chamber with a handle down form that?
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Unread postAuthor: clide » Fri Apr 20, 2007 4:06 pm

goathunter wrote:Clide,The gun has to be coaxial to be a MKX look alike.How did you build the check valve in the piston?And is the checkvalve better in larger pistons or just the smaller?


I understand the look he is going for, that kind of valve will work. Toolies guns are over/under to save space, but it will work great for an inline gun.

Check valves are most beneficial when you have a low flow exhaust. So bigger pistons would probably benefit more because a given exhaust valve would be relatively smaller for a larger gun.

surfbum PMed me to ask about the check valve so I already sent this to him, but I'll post it here too since you asked:

As for the check valve there are many ways, it doesn't have to be a perfect seal to work well. Some people use rubber flaps over holes. Some people use store bought ones. On my last few guns I made ones with a couple brass fittings, a ball bearing, and a pen spring.

The fittings are a 1/8" Male x 1/4" Female adapter and a 1/4" Plug. I drill a small hole in the 1/4" plug, and put the 1/4" ball bearing (or metal slingshot ammo since I already have a bunch and it's cheap) in the adapter so it sits where it reduces down to 1/8" then I put the spring on top of that and screw on the plug. Then I can just screw the 1/8" male into wherever I need it. Here are a few pics that will hopefully help explain.
http://gbcannon.com/pics/hsl/check.jpg
http://gbcannon.com/pics/hsl/piston1.jpg

I've been meaning to order a few of these and give this a shot. I would just cut off the barb and glue them into a hole.
http://www.usplastic.com/catalog/product.asp?catalog%5Fname=usplastic&category%5Fname=45&product%5Fid=20411
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