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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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Unread postAuthor: jackssmirkingrevenge » Sun Nov 27, 2011 5:57 am

A 3mm diameter sphere has a volume of 0.01414cm<sup>3</sup>, steel has a density of about 7.85g/cm<sup>3</sup>, so one BB weighs 0.11 grams.

I think I read on the Farco Air Shotgun website that the Farco got 300 fps


Feet per second values without projectile weight are irrelevant ;)
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Unread postAuthor: Zeus » Sun Nov 27, 2011 6:02 am

Well, if a 2.75 inch shell doesn't work for you, then how about 3.5 inch magnums? They feed in some semis, so designing a launcher for them won't be exceedingly hard.
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Unread postAuthor: DerpWithAGun » Sun Nov 27, 2011 6:59 am

jackssmirkingrevenge wrote:A 3mm diameter sphere has a volume of 0.01414cm<sup>3</sup>, steel has a density of about 7.85g/cm<sup>3</sup>, so one BB weighs 0.11 grams.

I think I read on the Farco Air Shotgun website that the Farco got 300 fps


Feet per second values without projectile weight are irrelevant ;)


Sorry It was 15 grams of projectile weight with 350 fps. :)

Anyway, I'm making a lever action so I don't think magnum rounds would work. The gun will look weird with such a large ejection hole. I may be able to increase the size by 1cm or so.

Btw JSR, do I need to drill any holes, cut grooves or add O-rings to the piston so the air can go past it to fill up the rest of the cartridge or can air just seep past because it is steel - steel contact?
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Unread postAuthor: dewey-1 » Sun Nov 27, 2011 12:46 pm

Is this what you are looking for?
3 inch long total, 3/4 inch OD.
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Unread postAuthor: DerpWithAGun » Sun Nov 27, 2011 3:45 pm

Wow dewey is that my design in 3D or a different design? It looks like mine.
If there is a shotgun shell in real life that is 3 inches long then I'll probably go with that or just increase my design by 1cm.
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Unread postAuthor: Zeus » Sun Nov 27, 2011 3:53 pm

There's 2.75", 3" and 3.5" shotgun shells available. Lever actions work with 3" shells, don't know about 3.5".

Don't forget that larger diameter shells are made too, like 10 gauge (which lever actions are available in), the shells are 3.5" long.
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Unread postAuthor: dewey-1 » Sun Nov 27, 2011 6:09 pm

DerpWithAGun wrote:Wow dewey is that my design in 3D or a different design? It looks like mine.
If there is a shotgun shell in real life that is 3 inches long then I'll probably go with that or just increase my design by 1cm.


The design was for a hybrid cartridge that I did for JSR and modified it for a pneumatic cartridge. Here is a wiremesh side view.
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Unread postAuthor: warhead052 » Sun Nov 27, 2011 6:12 pm

Not to hijack or anything, but how much would it cost per shell like that dewey?

I think it would work though, the only thing would be a firing mechanism (unless I missed that somewhere in the thread...)
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 27, 2011 10:01 pm

Very nice as always dewey!

(I've downloaded sketchup in the meantime... baby steps ;))

I would make the piston out of delrin, this would remove the need for a bumper and possibly seal, and greatly reduce piston travel. The chamber is so small that piston bounce should not be an issue.
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Unread postAuthor: DerpWithAGun » Mon Nov 28, 2011 12:24 am

So Is it better to make the piston out of steel or delrin? what are the advantages and disadvantages? Where can I buy Delrin? And what do you mean by piston travel?

I'm going to continue drawing more designs now. Having 70 BBs is a bit much for me since buying BB's from overseas may get confiscated. On Ebay Australia you can get a pack of 100 BBs that are 3mm in diameter for $9. If I was to do 70 BBs per load that is like $9 used in an instant. I may just go with 20 BBs per load.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 28, 2011 1:09 am

DerpWithAGun wrote:So Is it better to make the piston out of steel or delrin? what are the advantages and disadvantages?


Delrin is lighter (faster opening time and less stress on the launcher) and much, much easier to machine than steel, and is also potentially self-sealing in high pressure applications, meaning you might not need a rubber seal where it touches the "barrel"

Where can I buy Delrin?


Delrin is a trade name forpolyoxymethylene, if not Delrin people usually call it "acetal". You can get it online but it's much cheaper to find a local source, people who supply metal stock for machining should have a line of polymers too.

And what do you mean by piston travel?


Er... how much the piston can travel ;)

For a 10mm "barrel" in theory it doesn't have to move back more than 2.5mm for full flow, I would limit it to 4mm to account for some bounce.

I'm going to continue drawing more designs now. Having 70 BBs is a bit much for me since buying BB's from overseas may get confiscated. On Ebay Australia you can get a pack of 100 BBs that are 3mm in diameter for $9. If I was to do 70 BBs per load that is like $9 used in an instant. I may just go with 20 BBs per load.


As this is for a shotshell, precise BBs aren't required.

You're much better off buying this, half a kilo of ammunition for 17 dollars ;)
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Unread postAuthor: DerpWithAGun » Mon Nov 28, 2011 1:32 am

Alright thanks JSR,

I've finished my final designs of my cartridge and drawn in all the wall thicknesses. The wall thickness for all parts is 1mm thick. The whole cartridge has been increased to 3 inches in length. The piston will be made out of delrin and the piston's thickness has been increased to 1cm. (the piston no longer needs to overlap the barrel.) There is a 9mm gap behind the piston for pressure to build. This cartridge will take 1000 psi but I want to do GGDT tests with 1000 psi and 600 psi because if it is weak at 600 psi then I can't buy a 600 psi shock pump and I don't even know if this: http://www.ebay.com/itm/Benjamin-Hand-P ... 2de90#shId will even get 1000 psi on such a small object.

Thanks also for the link to the Shot, I'll probably buy 1 kilo. ;)
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 28, 2011 1:37 am

DerpWithAGun wrote:There is a 9mm gap behind the piston for pressure to build.


You want a small pilot volume for better efficiency, again assuming a 10mm barrel this definitely doesn't need to be more than 5mm

I don't even know if this will even get 1000 psi on such a small object.


Er... smaller object = high pressure quicker!

I think at 600 psi it will be more than adequate for your purposes. However if you can afford the stirrup pump it will fill it more quickly and with less effort.

One important thing to note is that the stirrup pump will not connect to a schrader, so some sort of adaptor will be required if you go with schraders on your cartridges.
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Unread postAuthor: DerpWithAGun » Mon Nov 28, 2011 1:59 am

Okay I will adjust my designs lightly more (reducing the gap behind the piston) since my piston is made of delrin do I need to have any holes, grooves or O rings on the outside of the piston so air can pass through or what?
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 28, 2011 2:14 am

DerpWithAGun wrote:do I need to have any holes, grooves or O rings on the outside of the piston so air can pass through or what?


No, it just needs to be a very close fit.

Might have a crack at this actually, for use with the fill rig. By simply adding an electrode it could potentially be used as a hybrid cartridge too. I'm not 100% sure the delrin only piston would work, thought there are plenty of HPA applications, for example the Talon/Stealth style valve uses delrin on metal contact to seal:

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