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Need Help with Interals of a Pneumatic LAW

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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Need Help with Interals of a Pneumatic LAW

Unread postAuthor: Airsoft Junkie » Sun Jan 07, 2007 7:51 pm

Hey Folks -

After spending a good portion of my weekend reading though various threads on the site I decided to register and post up some questions. I've been airsofting for about 6 years now, and I've decided I want to make my own rocket launcher. Specifically the M72 LAW, and I want to keep the aesthetics close to the real thing as possible.

Image
30.5" closed - 38.5" open - 2.5" barrel (business end to the right)

I plan on using the NERF Vortex footballs with the tail on them for ammo, hopefully with a range of 100 yards. It would be great to run it on 12g Co2 powerlets in the instant tire inflators. Pumping it up with a bike pump is always an option too. I'm also thinking about if I want to use a straight system, or if an over/under would work better. If I went over/under, the chamber would have to be small enough to fit in the boxy part on the main tube where the push trigger and sights are.

I'm also wondering if the following valve would work:
Image
Push Button Self-Closing 90 Degree Straight Valve

I worked with the GGDT to try and get an idea of the required sizes for barrels and chambers, but I didn't have much luck with it. I also took a good look at the post on chamber sizes for 12g Co2
http://www.spudfiles.com/forums/viewtopic.php?t=244&

But I'm not sure about how much PSI would be need to shoot a NERF at the ranges I'm looking for.

SO! Any sort of help, and/or suggestions would be great.
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Last edited by Airsoft Junkie on Sun Jan 07, 2007 7:54 pm, edited 1 time in total.
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Unread postAuthor: pyromanic13 » Sun Jan 07, 2007 7:54 pm

i'm co-axil... anyone else with me? and I'm sure you can buy the actuall gun without ammo obviously.. I mean I have a real one in my closet.
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Unread postAuthor: judgment_arms » Sun Jan 07, 2007 8:10 pm

I’ll have to run it through the GGDT but this is basically what you’d want to do:
Coaxial (barrel in the chamber search for it) with a 2inch barrel and a three in pressure chamber sleeved in 4inch, You put the cosmetics (the stuff that makes it look like a LAW) on the 4inch pipe and the guts go inside. If you’ve done as much research as you say you have then most of this makes since. Another, simpler, design is to make it an inline with a modded sprinkler valve and use a push button valve or an elbow and a blowgun. The only problem with an inline is that it’ll look more like a Bazooka. Also you will NOT be able to make it telescope, unless the other half is just empty which would be pointless as the gun it self would ether be too small or the whole thing would be to big open. I’ll let you think about this for a little bit well’s I run the two proposed guns through the GGDT.
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Unread postAuthor: Airsoft Junkie » Sun Jan 07, 2007 8:19 pm

Coaxial would be nice, but that'd make it way to big in diameter if I wanted to fire the NERFs which I'm guessing are about 2 inches in diameter themself. Which would bring the chamber into the 4 inch range like you described. It would kill the look I'm going for. :(
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Unread postAuthor: Bluetooth » Sun Jan 07, 2007 8:21 pm

Real LAWs are about 4" in diameter anyway.
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Unread postAuthor: Airsoft Junkie » Sun Jan 07, 2007 8:23 pm

Not according to:
http://world.guns.ru/grenade/gl12-e.htm

Caliber: 66 mm (2.59843 Inches)
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Unread postAuthor: judgment_arms » Sun Jan 07, 2007 8:28 pm

The first gun is the coaxial here’s the technical information:
Chamber: length = 4ft, diameter = 3in
Barrel: length = 43in, diameter = 2in,
Estimated muzzle velocity and energy with a 1oz projectile: velocity = 485fps, energy = 229.2ft-lb.
Estimated max range and angle required to achieve said range: range = 369ft or 123yards, firing angle = 27 degrees.
Evaluation: may be to powerful, 229ft-lb is enough to kill a man, literally, but that is the velocity required to reach out and touch somebody at 100+ yards. The thing could be made smaller, and safer, but it’ll decrease range. Other than that, this thing surprised me on how powerful it is I wouldn’t have thought that there would be a big enough chamber.

I’m going to eat now and run the other one through the GGDT and post that one.


Edit:
Didn’t see the other posts, so four inches is too big? What kind of performance do you want/need? Or why not make an AT-4? It’s larger in diameter, I think
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Unread postAuthor: MrCrowley » Sun Jan 07, 2007 8:40 pm

Your going to have to go 4", Whats the probelm? Most people who have done these designs go 4".
Or you can have one long 3" chamber but it will make the gun look funny because of the small difference in diameter of barrel and chamber.
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Unread postAuthor: Airsoft Junkie » Sun Jan 07, 2007 8:42 pm

Chamber of 4' and a Barrel of 3.5' - huh? That doesn't work... A piston isn't going to be 6 inches long.

I don't know how much the NERFs weight, but I would guess it'd be more then 1oz.

If it takes that much power to put it out 100 yards, there is no way field owners will let me use it. Safety, insurance, etc. take your pick. Sounds like I might have to scale down my ambitions a bit.
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Unread postAuthor: judgment_arms » Sun Jan 07, 2007 8:44 pm

I think this will be more to you liking, the whole thing’s only 4 foot long:
Chamber: length = 20in, diameter = 2in
Barrel: length = 24in, diameter = 2in,
Estimated muzzle velocity and energy with a 1oz projectile: velocity = 288fps, energy = 80.5ft-lb.
Estimated max range and angle required to achieve said range: range = 290ft or 96.666yards (uh oh, 666), firing angle = 32 degrees.
Evaluation: significantly smaller than the first one I posted with about the same range. Although, it may be to long, is it?

This one’s the inline by the way.
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Unread postAuthor: pyromanic13 » Sun Jan 07, 2007 8:57 pm

make a coaxil with a 2" barrel and 2.5" chamber. make it 2.5' long
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Unread postAuthor: judgment_arms » Sun Jan 07, 2007 9:03 pm

Sorry pyromanic13, but there’s not enough chamber volume there the OD of 2inch SCH40 PVC id almost 2.5inches...
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Unread postAuthor: pyromanic13 » Sun Jan 07, 2007 9:08 pm

whatever...2.75 (if it exists) or more probebly 3" or just a 3 foot piece of 2" pipe with 1/3 of it a chamber closed off by a gatevalve. I think those seal to like...60 psi.
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Unread postAuthor: judgment_arms » Sun Jan 07, 2007 9:22 pm

Or he could just make the second gun I posted, inline with a modded sprinkler valve, nothing’s simpler and as effective. Gate valve? The only gate valve I know of is like a faucet and is slower than a ball valve, like 30 seconds to open. I hope your talking about something else.
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Unread postAuthor: pyromanic13 » Sun Jan 07, 2007 9:44 pm

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