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.1835 C:B Ratio Acceptable for Pneumatics?

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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.1835 C:B Ratio Acceptable for Pneumatics?

Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 9:53 am

I'm working on a coaxial piston cannon with a 2.5 inch barrel and 3 inch chamber, at 48 inches long I calculated the volume of the chamber to be 43.25 square inches and the barrel to be 235.6, giving a B:C ratio of .1835. Is this within acceptable range? The reason I'm using 2.5 inch for the barrel is, first, so I can shoot tennis balls (obvious) but more importantly so that I can have a changeable barrel system.

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Unread postAuthor: inonickname » Wed Jul 13, 2011 10:07 am

What pressures and valve are you using, and what kind of performance are you looking for
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Unread postAuthor: Technician1002 » Wed Jul 13, 2011 10:07 am

Please correct the units of measure and maybe we can understand the problem. Volume is a cubic measurement. Area is a square units measurement. Don't send me to the store for 3 square inches of milk.
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 10:23 am

Haha yeah... Thanks Technician, sorry 43.25 and 235.6 cubic inches. As far as performance, I'm looking for, well, good performance. I certainly don't want to be limited by the lack of air to push it. I was looking on spudwiki and it said that according to GGDT the ideal was between .2 and .3, with changeable barrels as well I know that with a golf ball barrel and others it would be fine, I just want to know if it will do well with the tennis ball also.

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Unread postAuthor: jackssmirkingrevenge » Wed Jul 13, 2011 10:33 am

How do you plan changing barrels in a co-ax?

I would suggest you play with GGDT, it will let you know if the power range is acceptable for your requirements for the pressure you intend to work with.
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Unread postAuthor: Gun Freak » Wed Jul 13, 2011 10:33 am

.2 and .3 make no sense for a PVC pneumatic... You must have read it wrong... Performance probably wont be great in my opinion.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 13, 2011 10:35 am

Gun Freak wrote:You must have read it wrong... Performance probably wont be great in my opinion.


It depends on the other launcher parameters - if you have a burst disk, long barrel and high pressure, performance will be quite impressive even with a 0.2 C:B ratio.

Por ejemplo:
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 11:07 am

Thanks Jack, I would play with GGDT but I have no idea how to... And Gun Freak I am capable of reading, and it said "Suggestively, the optimal C:B ratio for pneumatic guns, as predicted by GGDT are in the vicinity of 0.2~0.3" Also Jack, once I get some rough plans drawn up I'll post a pic.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 13, 2011 11:22 am

Jabingo19 wrote:Thanks Jack, I would play with GGDT but I have no idea how to...


Download it and learn, it's worth the effort.

Here's a more realistic model of your coaxial, about as much power a 3 round burst from a 9mm submachinegun ;)
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 11:51 am

Also, I don't know if you have any experience or whatnot, but is some sort of barrel support at the bottom of the coax necessary? At the end of the barrel is it necessary to have that supported so it doesn't wobble when sealing against the piston?
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 13, 2011 11:57 am

Depends on how rigid the barrel is and how rigidly it is mounted, how long it is, how well centred it is, what pressure you're using...
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 11:59 am

Barrel will be about 4 ft in a piece of 4" PVC (3" just wasn't giving me enough room for my liking), 2.5 inch sch 40 (so fairly rigid) it will be piloted by a sprinkler, so no higher than 125-150 psi

Hope that helps
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 13, 2011 12:10 pm

4 feet is a bit much, I would put spacers. Could be as simple as three bolts each at 120 degrees from eachother at the breech, will also make a useful stop you prevent you from pushing ammunition into the chamber.
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Unread postAuthor: Jabingo19 » Wed Jul 13, 2011 12:14 pm

Alright thanks Jack. Rough plans to come up soon.
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Unread postAuthor: Gun Freak » Wed Jul 13, 2011 12:45 pm

The bolts have to be sealed though, or else air will leak out the barrel.
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