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Coaxil Piston Gun Questions/Suggestions

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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Coaxil Piston Gun Questions/Suggestions

Unread postAuthor: LCTChamp » Tue Aug 16, 2011 9:53 am

After building pretty much every type of gun besides a hybrid or a piston (I never get around to posting my guns lol), I think its time for me to try a piston gun.

Since I already have almost all the parts laying around I've already constructed the piston but nothing is glued yet (pics below).

Do you see anything wrong with the piston? It fits perfectly with a nice seal. I know I need an equalization hole but what size should the hole be?


My chamber will be 2" pvc approximately 30" long. I would like my barrel to be approximately the same length or just a little longer since im going for compactness rather than power. Should I use a 1.5" barrel or 1" barrel for a better C:B ratio? Also how much space should the piston move to make the seal against the barrel?

I will be running it at about 120psi since I just got a new compresser :D and will be piloting it with a 3/4" or 1"sprinkler valve (it depends which gun I want to dissasemble) piloted by a blowgun
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Unread postAuthor: mattyzip77 » Tue Aug 16, 2011 10:04 am

The piston looks good.As far as the hole, it has to be small. With my pistons, they are able to move freely like 2 to 2 1/2 inches. What do you plan to shoot?? I recommend sdr 21 1 1/2 inch pipe for golfballs.Also on your piston, you should put a steel washer behind the rubber sealing face washer. Gun Freak is the man to ask about this, probably Mr.Crowley too!! :D :D
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Unread postAuthor: LCTChamp » Tue Aug 16, 2011 10:17 am

I have 4ft of sdr 21 left in my garage right now ;) but do you think with only say 3ft of barrel my C:B ratio would be okay? What I want to shoot depends on what size pipe would give me the best ratio with a 36" barrel. If its 1" then batteries, 1.5" potatoes, sdr 21 golfballs. I will add that washer I completely forgot about that. Thanks for the help matty
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Unread postAuthor: mattyzip77 » Tue Aug 16, 2011 10:33 am

On my piston cannon, my chamber is about 30 inches of 2 inch and my barrel is 3 feet of sdr 21 1 1/2 inch. I get 400 yards easy with this setup. If you dont add the washer, your rubber washer will either rip or WRINKLE without it. Hope this helps. Good luck!! Also, you can always have interchangeable barrels!! :wink:
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Unread postAuthor: Labtecpower » Tue Aug 16, 2011 10:50 am

I would go for the SDR 21 barrel. It will have a chamber : barrel ratio of 1.7, wich is more than enough. I think it will be a bit noisy.
The C:B ratio will still be allright with 3' of pipe.

The EQ hole should be as small as possible.
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Unread postAuthor: Technician1002 » Tue Aug 16, 2011 12:01 pm

Please don't hate me.. My turn to rip the piston design. I'm not doing this to make enemies. Please consider my feedback as constructive criticism. Please...

My first impression on the piston is nice piston. Construction looks first rate. Now the bad impressions..

1 It is a hollow piston with an open back. I have had nothing but problems with open back pistons as there is not enough area for a proper bumper to do the job. My Mouse Musket and Dragon cannons ate pistons for lunch. My first QDV shattered the pilot housing. I later re-used the piston, but I added a washer to the back of the piston (not attached) so the bumper could not go up inside the piston.

2 Is that PVC? Other than the modified original QDV piston with the added washer and extended length bumper, all my PVC pistons eventually broke. PVC does not like the bumps and bruises of being a piston.

3 What is under the O ring groove.. The plastic under the o ring appears to be really thin. I would expect the piston to become broken at a very young age.. Nuf said.

4 Why is the o ring so far away from the back of the piston? When the piston sticks out of it's cylinder to extend out to the barrel seal, the o ring must remain tucked into the pilot. If it doesn't then it isn't working. Recommendation is to cut off the back of the piston to make it shorter, lighter and add more bumper and the washer to enclose the back of the piston.

Sorry for ripping the piston so badly. It looks great.

For an example of a small piston with the o ring near the back, see this thread.
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http://www.spudfiles.com/forums/tutorial-making-a-small-o-ringed-piston-on-a-drill-press-t21740.html

The EQ hole should be as small as you can make it.1/16th inch or smaller is recommended.

How far should a piston move? The best distance is a balance. If it moves too far, it picks up too much speed and breaks things. If it does not move far enough, it block the flow.

The minimum distance is generally considered 1/4 the distance that the opening (orifice) into the barrel size is. So a 1 inch orifice would have a piston move no less than 1/4 inch. This relationship provides a gap with the same open area as the area inside the opening to the barrel. i look at that as two equal restrictions to flow in series. 1 is the gap between the piston and barrel, and the second is the area of the entrance into the barrel. To minimise the restriction of one of those so only one significant restriction to flow remains, I like to design my pistons to move between 1/3 to 1/2 the barrel opening diameter instead, so in the above example with a 1 inch opening to the barrel, a piston motion of 1/3 to 1/2 inch will perform very well.
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Unread postAuthor: jor2daje » Tue Aug 16, 2011 12:13 pm

I could be wrong but I believe you only need the piston to travel back 1/4 the diameter of the sealing face for full flow.

Edit haha missed the bottom of tech's post, that's much better information.
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Unread postAuthor: mattyzip77 » Tue Aug 16, 2011 1:07 pm

Tech is right, I assumed that you would fill the piston with epoxy, and or cap it, and the o ring would be better toward the back of it. What do you plan to house the piston in, and how big is it??? Give us as much info as you can. The more info, the better we can help.
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Unread postAuthor: LCTChamp » Tue Aug 16, 2011 2:49 pm

No offense taken I posted this for criticism. It is pvc and I used thin wall for it to be light but apparently that was a bad idea. it could easily be capped/filled or both. Nothing is glued so the oring can easily be moved. since its a coaxial it will be housed in the 2" pipe and I was going to put a female adapter with a plug for easy removal or fixing.

Edit: Would hot glue work as a filler? I'm trying to work with what I have and I'm out of epoxy.
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Unread postAuthor: Mr.Tallahassee » Tue Aug 16, 2011 10:06 pm

I'm sure hot glue would work seeing as many people have made pistons entirely from hot glue. I recently made one and its doing quite well under all the stress of ~200PSI.
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Unread postAuthor: LCTChamp » Wed Aug 17, 2011 7:56 am

Mr.Tallahassee wrote:I'm sure hot glue would work seeing as many people have made pistons entirely from hot glue. I recently made one and its doing quite well under all the stress of ~200PSI.


That was my logic also. Thanks for the help everyone if theres any more pointers/criticism/tips let me know! You've already saved me from a bunch of unneccasary trial and error
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Unread postAuthor: MRR » Wed Aug 17, 2011 10:56 am

My suggestion is, turn the piston around, put the rubber washer into the piston and drill a hole near the outer edge. This way you have a very light, fast acting cup piston with build in check valve.
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Unread postAuthor: Technician1002 » Wed Aug 17, 2011 1:00 pm

One great way to reduce the piston weight is to reduce the diameter. The added benefit is the piston OD and the barrel seal diameters have a lower ratio. See my Mouse Musket thread in my sig for more information on that design. It is a 1 inch barrel in a 2 inch coaxial chamber with a 1.5 inch piston. A 1.5 inch piston is much lighter than a 2 inch piston. It was a little tricky to add the 1.5 inch pipe in the breech of the 2 inch chamber. It is not for beginners.
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Unread postAuthor: LCTChamp » Wed Aug 17, 2011 1:17 pm

Technician1002 wrote:One great way to reduce the piston weight is to reduce the diameter. The added benefit is the piston OD and the barrel seal diameters have a lower ratio. See my Mouse Musket thread in my sig for more information on that design. It is a 1 inch barrel in a 2 inch coaxial chamber with a 1.5 inch piston. A 1.5 inch piston is much lighter than a 2 inch piston. It was a little tricky to add the 1.5 inch pipe in the breech of the 2 inch chamber. It is not for beginners.


I'm going for simplicty here so I don't want to have to heat and ram a 1.5" female adapter in my 2" pipe (I think that is the only way to do what your saying since thats how your mouse musket was done) But that design does make the mouse musket look very cool and unique.

The piston has been shortened to allow the oring to be closer to the back, I added a large washer in between the cap and sealing face, next I will glue it together and fill it with hot glue. I will keep you all posted. And if this piston fails im only out say 10min work and no money since i had the parts lying around.

My only other issue now is a good bumper which will be absolutely necessary since my piston is pvc. Any ideas right off?
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Unread postAuthor: saefroch » Thu Aug 18, 2011 11:09 am

Anything rubber. People often use half a tennis ball or some coiled tubing in large PVC builds.
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