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2.4kpsi helium powered BB gun - build log

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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2.4kpsi helium powered BB gun - build log

Unread postAuthor: DYI » Fri Jan 04, 2008 1:50 pm

Well, yesterday the parts I ordered from McMaster for my BB gun arrived, which is quite a feat in itself, considering that McMaster won't ship to Canada, and the parts had to go through 3 different courriers to get here.

I got: 1" OD x 12" length PTFE round bar
1" OD x 12" length polycarbonate bar
4000 psi pressure washer handle
0.257" ID, 5/16" OD x 72" long stainless steel tube
5/16", 3000 psi Swagelok compression fitting

Some pics: Image

Image
As you can probably guess, the clear one is polycarbonate, the white one is teflon

Image
The pressure washer handle

Image
An airsoft BB in the barrel. Considering that they are .24, and my planned ammo is .25, the fit should be excellent. It takes more than 5 seconds for the BB to reach the other end of the barrel if it is dropped in when one end of the tube is plugged.

I'll keep updating this post as the work continues. The next part of the project is finding a machine shop to create the parts for the valve ($$$). The following is a diagram of the outer parts of the gun, and a rough approximation of what it will look like when it's finished.

Image
Another awesome paint pic, courtesy of LCWMD. Building airguns of dubious quality and questionable legality since 2007.
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Unread postAuthor: Carlman » Fri Jan 04, 2008 2:06 pm

wait, so that PW handle is a pilot for some sort of high pressure valve using 2.4k psi of helium that will in turn make a bb go frigin fast!! WOW

this is gonna be good. and that handle looks really good!
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Unread postAuthor: Hotwired » Fri Jan 04, 2008 2:11 pm

I went down to a local machine shop the other day and they charge £30/h for milling work.

A piston valve for 0.25 sounds like you'll be venting more gas through the washer handle than through the barrel :wink:
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Unread postAuthor: DYI » Fri Jan 04, 2008 2:17 pm

Yep, the PW handle is a pilot valve for the piston valve I'm getting built. It will also be equipped with a quick connect on the inlet, to allow its use to pilot other projects (as I'm not buying another $60 pilot valve any time soon).

I don't know if I mentioned it, but the planned ammo is .25" tungsten carbide ball bearings, which should weigh ~2grams, and travel at >2000fps.

@Hotwired: I opened up the pressure washer, and the flow is fairly constricted. The chamber volume will be almost 10 ci, and the pilot volume will be <1ci, so not quite more through the pilot than through the barrel. That, and the piston is only going to be .001" smaller than its casing. If that fails, then I'll have to add O-rings and an improvised check valve.

If I can get the machining done to these tolerances for 60CDA/hour, I'll be relatively happy. Hopefully not that much though, as I only make $120/week. (however in the summer, I'll be working a lot more)
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Unread postAuthor: octane89 » Fri Jan 04, 2008 2:18 pm

What kind of valve did you have in mind? I have a small lathe and MIGHT be able to help you out if you get outragous milling prices or something.
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Unread postAuthor: pizlo » Fri Jan 04, 2008 2:32 pm

wow this is really going to be a work of art. Cant wait to see what it will put a pellet through :)
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Unread postAuthor: DYI » Fri Jan 04, 2008 2:38 pm

@Octane: The valve I have in mind is a piston valve design. A simplified version can be viewed in this thread. If you are capable of reliably producing tolerances of .001" or better, and cutting threads that are free of defects and will seal at 2400 psi, I may contact you later.
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Unread postAuthor: ALIHISGREAT » Fri Jan 04, 2008 2:54 pm

wow i can't wait to see this its gonna be sooo powerful :twisted:

oh and what are the threads on the pressure washer handle?
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Unread postAuthor: DYI » Fri Jan 04, 2008 3:11 pm

The inlet and outlet threads are both 3/8" NPT.

It won't be particularly powerful (~280ft/lbs), but it will be reasonably high velocities, and I expect some relatively impressive results from the tungsten carbide BBs.
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Unread postAuthor: POLAND_SPUD » Fri Jan 04, 2008 3:15 pm

lol tungsten carbide BBs ? that's gonna be expensive...
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Unread postAuthor: DYI » Fri Jan 04, 2008 3:32 pm

The carbide BBs are ~$15 for 5. They obviously won't be used as commonly as the steel bearings (of which several hundred can be bought for the same price), but they will achieve higher energy, and deform far less (if at all). On a vaguely related sidenote, the carbide BBs have a diameter tolerance of 1/10 000th of an inch, and a sphericity tolerance of 25/1 000 000ths of an inch.
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Unread postAuthor: POLAND_SPUD » Fri Jan 04, 2008 4:09 pm

you can always use lead instead - it's not as heavy as tungsten but much cheaper... it's worth considering becasue BBs can get lost easily after firing
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Unread postAuthor: Ragnarok » Fri Jan 04, 2008 4:11 pm

DYI wrote:It won't be particularly powerful (~280ft/lbs), but it will be reasonably high velocities, and I expect some relatively impressive results from the tungsten carbide BBs.

That is still a huge amount for that bore, as a round of .22 LR is only about 110 fpe.

I'd be interested to see what it could do if you were to make a little spitzer bullet out of 1/4" steel rod. I reckon a 6 gram round would still do over 1500 fps, which would be towards a very impressive 500 fpe, enough to compete with even the most serious hunting air rifles for power.

Looking forward to seeing it.
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Unread postAuthor: Novacastrian » Fri Jan 04, 2008 4:13 pm

Glad to see you got all your bits, the cannon looks damn sexy in the picture you drew.
I think your best bet with the bits you need machined is this.
Turn up at the machine shop at knock off time and find a machinist, have your drawings with you and your bits, very kindly ask said machinist what he drinks (not coke) and offer to pay him in that. Like 2 bottles of Whiskey or 2 Cartons of beer for his services, chances are a thirsty bloke that has just finished work will take you up on your offer, saving you about 100bucks in the process.

So when you pulled the pressure washer handle apart did it seem like to you that it would seal, or do you think you may have a problem with it leaking?
Greatest of luck with the build DYI.
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Unread postAuthor: Ragnarok » Fri Jan 04, 2008 4:26 pm

Novacastrian wrote:Like 2 bottles of Whiskey or 2 Cartons of beer for his services, chances are a thirsty bloke that has just finished work will take you up on your offer, saving you about 100bucks in the process.

That might work - but only if you are old enough to acquire such beverages.

I can't recall how old DYI is, nor do I know the Canadian drinking age, but I do know that most people couldn't pull off said trick.
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