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Barrel to Chamber Ratio

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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Barrel to Chamber Ratio

Unread postAuthor: millons70 » Fri Apr 26, 2013 7:43 pm

My barrel is 1.5inch sdr21 4 feet long. I'm going to be shooting golfballs, most likely using a ball valva. (It's the only thing i can get my hands on as of now.) What would be the best sized air chamber for optimal results? I'm going to be using a bike pump to fill this up, so I didn't want to make a crazy sized chamber that has the same output as a smaller one.

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Unread postAuthor: Technician1002 » Fri Apr 26, 2013 8:04 pm

Use the GGDT to model it. Gas Gun Design Tool. Google it. Your optimum chamber size will be related to the inverse of the pressure used. For tire pump operation, I found that chamber volume = barrel volume works very well. To save on pumping, a chamber about 1/3 the size will do a decent job, but only if you use pressure above about 50 PSI. At lower pressure you will want a normal size chamber.
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Unread postAuthor: millons70 » Fri Apr 26, 2013 8:14 pm

I'll look into GGDT, but for my 4 foot 1.5inch barrel, I plan on using 4 feet of 2inch plus 2 2inch tees. What distances should i expect from it at around 80psi(I'm a fat boy so I can get it to decent pressure easily)

I've seen GGDT before and was completely confused by it.
If someone has spare time, mind computing some numbers for me?
It'll be a 2inch ball valve, ammo is golfballs.
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Unread postAuthor: mattyzip77 » Fri Apr 26, 2013 11:24 pm

You should get at least 300 yards.....
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Unread postAuthor: millons70 » Sat Apr 27, 2013 7:07 am

That would be great for +300. I'm goinng to be shooting this at a golf course near my house and the range only goes to 300. Everyone will be looking at me up there though, so I'm going to try and make sure it fits in a golf bag
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Unread postAuthor: BenMara » Wed May 01, 2013 5:47 pm

the HGDT/GGDT is for windows, wont work on my mac.. whats a better ratio 2.5:1 or 3:1 i already have my barrel length just need
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Unread postAuthor: Zeus » Wed May 01, 2013 9:58 pm

1.5:1
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Unread postAuthor: jackssmirkingrevenge » Thu May 02, 2013 2:52 am

BenMara wrote:the HGDT/GGDT is for windows, wont work on my mac.. whats a better ratio 2.5:1 or 3:1 i already have my barrel length just need


What type of valve are you using with what calibre at what pressure?

These are all important parameters to consider when thinking about chamber ratios?

If you're using 500 psi you obviously need a smaller chamber than 50 psi. If your valve is slow or low flow, then a large chamber volume will be wasted as most of the air would come out after the projectile has left the muzzle.

One should also consider factors like noise and ease of pressurisation when thinking about chamber size.
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Unread postAuthor: BenMara » Thu May 02, 2013 3:06 am

rainbird 1", tennis ball projectile, 100psi max, bike pump to pressurize,most of the time, but also have 15lb CO2 bottle, Noise? i wanted to make this launcher cause ammo is currently overpriced and 22lr is hard to find.

i already have 36" barrel and was thinking of using 4" pvc for pressure side
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Unread postAuthor: jackssmirkingrevenge » Thu May 02, 2013 3:11 am

I would go for something between 1:1 and 2:1, I don't have access to GGDT from here but you could make a modelling request in this thread maybe some kind soul can indicate the sort of performance you can expect: http://www.spudfiles.com/forums/ggdt-hg ... 23256.html
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Unread postAuthor: mika245 » Mon May 20, 2013 3:14 pm

Hello! I'm new guy in here and wondering if my air cannon barrel is too small compared to my chamber. My barrel is 200cm long and 20mm diameter. Barrel volume is 0,6 liter and chamber volume is 2,43 liter(extinguisher). How much can I enlarge my barrel to get max power?

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Unread postAuthor: jackssmirkingrevenge » Mon May 20, 2013 3:40 pm

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