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Cartridge Powered Air Rifle Ideas

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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Cartridge Powered Air Rifle Ideas

Unread postAuthor: DerpWithAGun » Fri Nov 25, 2011 1:58 am

Hey Guys,

I have decided to make an air rifle that shoots using cartridges. It seems to be efficient enough. So here is a simple drawing of my design of the Cartridge.

Image

Black = Steel or main material (undecided yet as what to use)
Grey = Piston
Green = Spring
Pink = Rubber O rings
Brown = Sealing face

The idea is that when filled with air, there will a hammer in the gun that will shoot forward and hit the back of the schrader valve that is there. ( I want small Cartridges so I'm thinking of a homemade Valve.) Some of the air will escape from the back sending the piston shooting backwards. The O-rings on the piston make sure that air can pass past them to fill up the cartridges but cannot escape backwards. The air rushes out the barrel sending the projectile flying.

This design still needs work. I will soon have access to a lathe and it will be fairly simple to make. I am new to this sort of design.

Please, tell me what you think of my design! :)
All advice is appreciated!
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Unread postAuthor: wyz2285 » Fri Nov 25, 2011 4:08 am

For me it looks ok, hope it works.
But to get a high performance, you probably needs high pressure.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 25, 2011 4:35 am

Image

Here's a possible design, grey parts are metal, purple parts are delrin or similar high strength polymer. Everything is epoxied in place. This means it is not serviceable if something goes wrong internally, but it's so simple that there's very little to go wrong.

By using existing tubing for the cartridge body and "barrel", you'll save a lot of work and money on materials.

By using a schrader a fill/pilot valve, you don't need to build a separate fill rig.

By using high pressure, you probably won't even need a rubber seal, the delrin/metal contact should be airtight as long as both surfaces are polished and true.

You can get a 600 psi shock pump for around 50 of your finest Australian dollars.
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Unread postAuthor: DerpWithAGun » Fri Nov 25, 2011 8:25 am

Hmm ok,

Using that 600 psi shockpump, does it take a lot of effort to pump something up to that pressure? Does it take long?

I'm thinking of making these cartridges the same as the 1/2" size PVC pipe except I will increase the thickness of the wall by 5mm. I will then create 2 end caps for it using the lathe. Does anyone know the maximum pressure rating for epoxy?

The only thing I don't like about the design is the use of the schrader valve. By schrader valve you mean the little valve that comes off the bicycle inner tubes? Unless you can get shortened version of the valve then I don't know. It will look ugly on the cartridge and because of the valves length it will stick out so much that the gun probably wont function properly. It's really hard to explain what I'm thinking here. It's late and I want to go to bed, I'll draw a picture of what I mean tomorrow.
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Unread postAuthor: pneumaticcannons » Fri Nov 25, 2011 8:38 am

Are you thinking of using pvc for the cartridges or just using something of the same diameter. I personally wouldn't trust 1/2 pvc at 600 psi. If you drop it...

As for the epoxy, never use the quick set type, thats the weakest. I normally use 1 hour epoxy(not sure what brand) I've heard that they even sell "pressure rated epoxy". However, that doesn't mean that a 2mm wall of epoxy with hold what the package says...
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Unread postAuthor: evanmcorleytv » Fri Nov 25, 2011 9:17 am

pneumaticcannons wrote:Are you thinking of using pvc for the cartridges or just using something of the same diameter. I personally wouldn't trust 1/2 pvc at 600 psi. If you drop it...

As for the epoxy, never use the quick set type, thats the weakest. I normally use 1 hour epoxy(not sure what brand) I've heard that they even sell "pressure rated epoxy". However, that doesn't mean that a 2mm wall of epoxy with hold what the package says...


um.. I wouldn't trust ANY PVC at 600 PSI!! That's F-ing crazy! Use THICK steel or copper pipe. NOT the stuff you buy at home depot. Go to your local plumbing store for high pressure gas pipe, like I do! And as for the epoxy, you are correct, but if you go the safe way (steel gas pipe) you'll probably need to use epoxy on the inside, then solder on the outside.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 25, 2011 11:11 am

DerpWithAGun wrote:Using that 600 psi shockpump, does it take a lot of effort to pump something up to that pressure? Does it take long?


It takes some effort but you can pressurise a small volume (say 10cm<sup>3</sup>) in under a minute. You adolescents are used to making vigorous use of your arms anyway ;)

If you want easier, make your own stirrup pump or get a SCUBA/paintball tank. An 8 litre SCUBA tank filled to 4500 psi will fill a small cartridge to say 750 psi thousands of times before it needs to be refilled.

I'm thinking of making these cartridges the same as the 1/2" size PVC pipe except I will increase the thickness of the wall by 5mm. I will then create 2 end caps for it using the lathe. Does anyone know the maximum pressure rating for epoxy?


Used properly, epoxy alone is enough. These hybrid cartridges were subjected to a couple of thousand psi of hot gas from hybrid combustion, without failure - and all I used was 5 minute epoxy.

The important thing is correctly mixing the epoxy, and properly roughening the contact surfaces. In the case of the cartridges I machined some grooves in the plug to improve adhesion.

The only thing I don't like about the design is the use of the schrader valve. By schrader valve you mean the little valve that comes off the bicycle inner tubes? Unless you can get shortened version of the valve then I don't know. It will look ugly on the cartridge and because of the valves length it will stick out so much that the gun probably wont function properly.


This was the easy option, a homemade valve plus fill rig means smaller neater cartridges.

um.. I wouldn't trust ANY PVC at 600 PSI!! That's F-ing crazy!


He said he'd be using the same size as PVC pipe, not actual PVC pipe.

Besides...

;)
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Unread postAuthor: Hawkeye » Fri Nov 25, 2011 4:44 pm

Delrin is very difficult to bond with Epoxy and the best bond achieved is not really up to high pressure.
I can't help thinking it would be pretty easy to hmake a metal cartridge that just threads together.
It's really just a piece of pipe with two plugs for each end that could be threaded in with some threadlocker to completely seal it.
The scrader end could have a better homemade substitute and it could be recessed into the body of the cartridge a bit so that just the "firing pin" is proud of the casing.
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Unread postAuthor: DerpWithAGun » Fri Nov 25, 2011 7:00 pm

Okay, so it's easier to have a schrader valve then to make your own one because you would have to create a filling rig. Is there a way I can shorten the schrader valve without damaging the internals? Well, If making a filling rig and a homemade valve is easy then I'm going to do that. I really do want smaller neater cartridges. I'm thinking 4cm long and 2cm diameter with a wall thickness of 3mm or 4mm. The small barrel inside the cartridge will be 1.5cm long.

Would someone be able to send me a picture of the design for the fill rig and the homemade valve on the cartridge? Then I'll have a better idea on how this works.

Thanks for the help so far guys!
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Unread postAuthor: Gun Freak » Fri Nov 25, 2011 7:09 pm

Making a homemade valve an fill rig is more work than using schraders. What's leading you on to the homemade route?
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Unread postAuthor: Hawkeye » Fri Nov 25, 2011 7:16 pm

Haven't been doing this stuff for a while so I'll have to dig out my things . A tiny one way valve is very easy to make with the added benbefit that you can just thread the cartridge directly to a Girandoni style pump.
Make a small half inch copper pump and you can get very high pressure without the clutter of hoses and check valves that most homeade pumps seem to have.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 25, 2011 7:17 pm

Hawkeye wrote:Delrin is very difficult to bond with Epoxy and the best bond achieved is not really up to high pressure.


jackssmirkingrevenge wrote:Used properly, epoxy alone is enough. These hybrid cartridges were subjected to a couple of thousand psi of hot gas from hybrid combustion, without failure - and all I used was 5 minute epoxy.


I can't help thinking it would be pretty easy to make a metal cartridge that just threads together.
It's really just a piece of pipe with two plugs for each end that could be threaded in with some threadlocker to completely seal it.


Threads are a bit of pain in the arse to cut, at least for me, so I prefer to avoid this.

I've made cartridges with recessed schraders as Hawkeye suggested:

Image

They can be cut down quite a bit.

Image

Image
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Unread postAuthor: DerpWithAGun » Fri Nov 25, 2011 7:37 pm

Gun Freak wrote:Making a homemade valve an fill rig is more work than using schraders. What's leading you on to the homemade route?


Homemade is smaller and neater, schrader valves are are too long and they stick out which will probably make it harder for the weapon to function.
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Unread postAuthor: DerpWithAGun » Fri Nov 25, 2011 9:20 pm

Okay guys I'm going to be using a schrader valve. I just found one of my spare ones that I cut off of my old bike. I cut it down with a saw and it's now only 2cm long. This is a good size and my end plugs will be 1cm deep so only 1cm of the valve will stick out the end, that's a good size. None of the internals are ruined.

I was just searching through my collection of empty firearm shells and found a .308 and a 12 Gauge shell. The 12 Gauge didn't have a primer inside so I shoved the schrader valve in the hole and it fit perfect. So I was wondering if anyone knew how to tap out the primer from a .308 because I was thinking It would be cool to use the same dimensions of the 12 Gauge shell and .308 round to build these airgun cartridges. (I hope it isn't banned to talk about firearm shells).
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Unread postAuthor: Zeus » Fri Nov 25, 2011 10:34 pm

As you said it's empty, just get a flathead nail with a 1.5mm or smaller diameter shaft, drop that through the flash hole, use a rod or something to smack it out.

Easy as.
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