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Piston shotgun build help

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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Piston shotgun build help

Unread postAuthor: Marffy » Mon Nov 19, 2012 12:05 am

Its been a while and i'm thinking of making a new gun. I wanted to make a shotgun type co-axial? piston gun. With a 3/4" steel tube and a schrader valve as a pilot valve, i'm wondering if the schrader valve will work as a good pilot? or can i use something bigger that will hold around 300psi that isn't a ball valve. A blowgun maybe? Also, where might i be able to buy some pipe thats about .22 cal for the barrel? that will also hold around 300psi

thanks for any help
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Re: Piston shotgun build help

Unread postAuthor: jackssmirkingrevenge » Mon Nov 19, 2012 2:20 am

Marffy wrote:With a 3/4" steel tube and a schrader valve as a pilot valve, i'm wondering if the schrader valve will work as a good pilot?


Assuming your piston is tight and you don't have too much pilot volume, it should be more than fine.

or can i use something bigger that will hold around 300psi that isn't a ball valve. A blowgun maybe?


This type of blowgun is usually fine up to at least 450 psi:

Image

Also, where might i be able to buy some pipe thats about .22 cal for the barrel? that will also hold around 300psi


Do you want a launcher that looks like a shotgun, or one that actually fires multiple projectiles like a shotgun? In the latter case, 0.22" is far too small a calibre for useful power. I would want 0.5" at least.
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Unread postAuthor: Marffy » Mon Nov 19, 2012 5:56 pm

I'm looking for it to act like and look like a shotgun, but a muzzle loader. Basically its going to be around 2 feet of 3/4 pipe on a wood stock. thinking about it .22 might be a little small, but i want something that can fit a slug type projectile (a pellet) and to shoot shot (#9 shot maybe) But i also want a caliber that won't kill the chamber volume.

As for the blowgun, is 450psi safe? they say 90psi max on them...
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Unread postAuthor: MrCrowley » Mon Nov 19, 2012 6:00 pm

is 450psi safe? they say 90psi max on them...
Yes, it's safe. Many people have taken them up to 300PSI or higher without trouble. If anything, the seals will fail and possibly cause the gun to fire. As long as you adhere to typical gun safety practices it shouldn't be a problem if the seals fail and cause the gun to fire as the gun will be aiming in a safe direction.
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Unread postAuthor: Marffy » Mon Nov 19, 2012 6:54 pm

MrCrowley wrote:Yes, it's safe. .


alright cool, i was just worried it the trigger would blow up in my hand.

Now, does anyone know where i can get some pipe of a certain small caliber that will hold up to the pressure?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 20, 2012 2:04 am

Marffy wrote:I'm looking for it to act like and look like a shotgun, but a muzzle loader. Basically its going to be around 2 feet of 3/4 pipe on a wood stock. thinking about it .22 might be a little small, but i want something that can fit a slug type projectile (a pellet) and to shoot shot (#9 shot maybe) But i also want a caliber that won't kill the chamber volume.


If you're using a shock pump, 2 feet of 3/4" pipe will take forever to get up to 300 psi. Don't forget that HPA pneumatics do well even with small volumes.

For example: http://www.spudfiles.com/forums/12-bore ... 21346.html

In this case a bigger bore of at least 10mm will serve you well.

Marffy wrote:As for the blowgun, is 450psi safe? they say 90psi max on them...


That's because they have no ambition ;) as MrC says, the construction can take the pressure easily, it's more the seals that might go but in my experience 300 psi is perfectly fine.
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Unread postAuthor: Marffy » Tue Nov 20, 2012 6:20 pm

jackssmirkingrevenge wrote:If you're using a shock pump, 2 feet of 3/4" pipe will take forever to get up to 300 psi. Don't forget that HPA pneumatics do well even with small volumes


I was thinking a normal compressor for testing, and co2 cartridges/ regulated hpa tank for power.

Still have no idea where to get a barrel
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 21, 2012 12:52 am

You local hobby shop should carry a selection of brass and/or aluminium tubing which will take the pressure easily, I have for example used 0.5mm wall thickness 6mm brass tubing in 800 psi coaxials.

Herewith an example: http://www.spudfiles.com/forums/mini-0- ... 20101.html

One important thing to note is that a thin walled barrel used at high pressure will over time dig into your piston like a cookie cutter.

Here for example I wrapped masking tape around the breech end then impregnated it with epoxy in order to protect the piston seal by giving it a wider area to push on and therefore prevent it digging in.

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Unread postAuthor: Marffy » Wed Nov 21, 2012 11:40 pm

jackssmirkingrevenge wrote:One important thing to note is that a thin walled barrel used at high pressure will over time dig into your piston like a cookie cutter.


My other piston cannon has that problem. every other shot it either cuts the seals or the piston cracks in half. The tape is a good idea however i might use an epoxy, now i just have to find a tube long enough, i have seen those tubes at my local store but they are only a foot long.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 22, 2012 2:04 am

Marffy wrote:The tape is a good idea however i might use an epoxy, now i just have to find a tube long enough, i have seen those tubes at my local store but they are only a foot long.


Appropriately sized drill bit to keep them centred and you can easily epoxy two one foot lengths together with a sleeve over the join.

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Unread postAuthor: Marffy » Thu Nov 22, 2012 12:32 pm

looks like a pain, and i might get the barrel crooked. i'll just hope to find a longer tube at a different hardware store i'm going too, If that fails i'll have to do that /\

i could possibly sleeve the two barrels together too
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Unread postAuthor: Zeus » Thu Nov 22, 2012 6:09 pm

My local sells telescoping tubing, so sleeving is obscenely simple. And IIRC, you're 'Strayan, so look at the chain hardware stores, they sell telescoping Al tubing up to 24mm OD.
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Unread postAuthor: Marffy » Sat Nov 24, 2012 7:57 pm

Ran into a problem today. Where the barrel meets the piston, The barrel wobbles around there. I think i need some kind of a brace to keep it from wobbling but it needs to let the air flow thru at the same time. What have you guys used to stop this?
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Unread postAuthor: Zeus » Sun Nov 25, 2012 2:55 am

It's just a support, look for something that fits the OD of the barrel, and the ID of the chamber. A cotton reel is one idea, but there's heaps you can use. I just jam some Al tubing in the gap and epoxy it.
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Unread postAuthor: Marffy » Sun Nov 25, 2012 4:17 pm

Zeus wrote:It's just a support, look for something that fits the OD of the barrel, and the ID of the chamber. A cotton reel is one idea, but there's heaps you can use. I just jam some Al tubing in the gap and epoxy it.


alright, i tryed using wood hot glued to the barrel, but when when i threaded the barrel in the seem in the chamber just ripped the wood off. i'll need to make a support that goes on after the barrel is threaded in
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