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New Kiwi Gun Advice

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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New Kiwi Gun Advice

Unread postAuthor: microman171 » Sat Nov 29, 2008 8:24 pm

My Bazookair cannon just grew mould. I dont know if PVC is affected by this or not, but my pipe was very old and yellowish.

So I have decided to build a completely new spudgun. I dont have a budget but I would like to stay under $100. I live in New Zealand so $100 isnt a particularly high budget.

I have no preference to the type other than no piston co-axials. I have built one and that was great (Bazookair), but I want to try something new. I would, however, consider a diaphragm co-axial.

Currently I have no materials other than clear PVC primer and Yellow PVC Cement.

I think copper would make a nice cannon but I dont have a high PSI source (100psi max), and copper is even more expensive than PVC.

Feel free to post some helpful advice :-)

Microman171
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Unread postAuthor: MrCrowley » Sat Nov 29, 2008 8:37 pm

Copper would be out of the question for that budget unless you stick to like a 3/4" chamber.

A meter of 50mm pressure rated PVC costs about $20 at most plumbing merchants I go to. The average 50mm fitting in PVC costs about $11-$15.

You probably could make something for just under $100, but it'd be pretty basic in the terms of co-axials.

80mm(3") pipe is out of the question for your budget. It costs over $30 a meter I think and the average fitting is easily $20-$25 each. Sometimes over $50.

So you'll be stuck to a maximum of 50mm for the chamber diameter. If you have this in co-axial form, you'd probably want to stick to 32mm and below for barrels.

Then your pilot valve will probably end up being a 1/2" ball valve for ~$12. As a solenoid valve will set you back $40-$45 no problem.

Then you still need your fill valve (QC or schrader), various brass fittings, pressure gauge etc... It's a pretty tight budget.
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Unread postAuthor: microman171 » Sat Nov 29, 2008 8:51 pm

Yeah I thought it might be. How about some kind of diaphragm (or piston, I dont really know which is better) 15mm? That was It shouldnt cost too much? And have reasonable power?

My 25mm Barrel, 50mm Chamber wasnt exactly great. I am hopeing for some good power, not a high caliber. I used to have a Coke bottle ball valve and that was great with 1/2 inch grapes.

I think with my budget, 1/2" woud be a fine barrel, as long as I can get some good power out of it.

If cost would permit, interchangable barrels would be cool, but not necessary.
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Unread postAuthor: MrCrowley » Sat Nov 29, 2008 10:13 pm

microman171 wrote:Yeah I thought it might be. How about some kind of diaphragm (or piston, I dont really know which is better) 15mm? That was It shouldnt cost too much? And have reasonable power?

My 25mm Barrel, 50mm Chamber wasnt exactly great. I am hopeing for some good power, not a high caliber. I used to have a Coke bottle ball valve and that was great with 1/2 inch grapes.

Sounds like your piston/diaphragm wasn't very well made. Pistons should never even come close to comparing with most ball valve guns in terms of power.

A 2" chamber is over kill for 15mm barrel, a 1.5" one would be just as good. A 1m chamber and a 1.2m barrel should have pretty damn good power with a piston or diaphragm. My favourite ammo with a 15mm barrel are paintballs and fishing sinkers.

Check trademe for PVC pipe as well, I nabbed 6m of 20mm and 6m of 15mm pressure rated PVC for $6 each incl delivery.

I'd go with a piston though, I just find diaphragms harder as you have to use a union with a co-axial diaphragm valve I think.
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Unread postAuthor: microman171 » Sun Nov 30, 2008 12:49 am

Yeah I wondered about those diaphragm valves. Maybe if I was American I would give it a go...

Is there any maths i can do to determine power when volume, pressure, etc is given?

15mm barrel with a 1.5" chamber sounds good :-).

Time to start figuring out prices!

-Microman171
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Unread postAuthor: MrCrowley » Sun Nov 30, 2008 1:16 am

http://www.thehalls-in-bfe.com/GGDT/faq.html

Just download that. All the terminology and help with the various parameters and variables on the program can be found on that website too. So if you have a problem with it, check their first.
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Unread postAuthor: microman171 » Sun Nov 30, 2008 3:19 am

Okay here are the figures and the stats:

Chamber
Gas: Air
Temperature: 70F
Pressure: 100psi
Outer Diam: 1.5"
Inner Diam: 0.75"
Length: 40"

Valve Data
Valve: Barrel Seal
Flow Coef: 45% (I wasnt sure on this one)
Seat Diam: .75"
Dead Volume: 0"3 (according to the FAQ co-axials have next - to no dead volume?)
Piston Diam: 1.5"
Piston Mass: 1 oz ( I played around with this one and it didnt - change anything?)
Vent Diam: 0.2" (I think this is the Diameter of a blowgun?)
Pilot Volume: 1.8" (If I have 1inch of extra space including - bumper)

Barrel Data
Bore: .23"
Length: 47"

Projectile Data
Friction: 0.5 psi
Mass: 5 gram (small grapes)
Diameter: .6"
Initial Posistion: 0" (not exactly 0 in but close)

Results
Travel time: 12ms
Pressure Drop: 21.4psi
Muzzle Energy: 60.4ft*lb
Muzzle Velocity: 594 ft/s



I dont know how realistic these results are but imagine a grape at 594ft/s, that's 650km/h!

If this gun cost less than $100 to build I will be very please to say the least!

-Microman171
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Unread postAuthor: MrCrowley » Sun Nov 30, 2008 3:24 am

Flow Coef is probably closer to 30-35%
Dead volume - Correct
Piston Mass - As long as it doesn't weigh like a kg or two, it'll be fine
Vent diam - .1 is a blow gun, .2 is a modded blow gun I think. I'd recommend at least a 1/4" ball valve, 1/2" or 3/4" would be better though. A blow gun wont cut it.
Pilot volume - Don't worry too much about this, as long as it's not too big you should be fine.

Bore: If you're going with a 15mm barrel, it'd be .75", you've already said that the seat diameter is .75" anyway.

Proj Diameter: Again, if you're going with a 15mm barrel, you'd find grapes as close as possible to the diameter, so probably .75" again.

Velocity sounds a bit optimistic maybe (flow coef is too high), it'd be around that range though.
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Unread postAuthor: microman171 » Sun Nov 30, 2008 3:49 am

The barrel diameter was supposed to be .6" (I was trying out BB diameters too)

After re-adjusting the parameters I still get >500 ft/s.

Now with your knowledge, how much do you think this gun my set me back?

    1m 15mm Pressure Pipe
    1m 40mm Pressure Pipe
    40mm x 15mm Pressure Reducer Bushing
    40mm Pressure Female Adapter
    40mm Pressure Male Thread Plug
    1/4" x1" brass nipple
    1/4" T
    1/4" Ball Valve
    Schrader Valve (Hopeing this is 1/4" too)


I dont need a gauge as my pump has one. I am not asking for an exact pricing, just a guestimate. Then I can either make cut backs, or I can start saveing and get everthing priced up.

Thanks for all of your help MrCrowley :-D

-Microman171
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Unread postAuthor: MrCrowley » Sun Nov 30, 2008 3:57 am

1m 15mm Pressure Pipe - $10
1m 40mm Pressure Pipe - $18
40mm x 15mm Pressure Reducer Bushing - $10
40mm Pressure Female Adapter - $10
40mm Pressure Male Thread Plug - $10
1/4" x1" brass nipple - You'll have to go 1"x1/2", then 1/2" by 1/4", this will cost probably $15 for both.
1/4" T - these are quite hard to find, i've only seen them online for about $16.
1/4" Ball Valve - these are like $8, i think there's one on trademe. I'd go with 1/2" though. It'll save you money as you wont need the 1/4" tee.
Schrader Valve (Hopeing this is 1/4" too) - A threaded one, i've never seen in NZ. I got mine in America. You'll have to use a Quick Connect for an air compressor, they're about $8


All prices from a plumbing merchant, just look one up in the yellow pages, they'll have a trade place out back. Master Plumber or similar are usually good.
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Unread postAuthor: microman171 » Sun Nov 30, 2008 1:11 pm

Wow Thanks :-). So a merchant will be better than Bunnings andthe like, or an actual plumber?

I dont have a quick connect, but I do have a 3/4" threaded schrader. I have epoxied a valveless car tire one in.

I also dont have an air compressor, but maybe I could get another car tire schrader, and epoxy it to the outside? Not so sure on that. I do have a very limites set of brass parts, female 3/4" T, Various Nipples, 1/2" ball valve if I do some 'grave robbing' and steal the one off the Bazookair.

Would it be better if I used a threaded reducer instead of a plug? That way I can screw parts in easier? Those threaded plugs are 'deep dish'

I would get the 40mm to 20mm?(3/4") pressure threaded reducer.

Once again thanks for all of your help :-)

-Microman171

---[UPDATE]---
I have threaded together the brass fittings needed for the exhaust. I now have a 3/4" pilot!

What would make an ideal piston? Remembering I have (up for change) a 1" x 1.5" pilot volume, and the bumper needs to go in there too. I was thinkings the same shape as the last one, just maybe a smaller aersol cap? My previous shape was a deoderant lid with the sealing face on the inside.
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