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C:B ratio question

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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C:B ratio question

Unread postAuthor: lockmanslammin » Sat May 10, 2008 10:03 pm

OK, so I did a search on this and found alot of opinions and numbers.
Here is what I'm curious about: When people ask the forum what the correct C:B ratio is, shouldn't on of the questions to the poster be "what pressure is the gun going to be used at?" Since upping the PSI in the chamber is also upping the volume of air in the chamber. I'm sure there has already been extensive discussion on this topic, but my search didn't un-earth it. So people say that a rule of thumb is to go between .5:1 and 4:1, what pressure are they at. A better way would be to say "what would be the best amount of PSI left in the gun when the projectile is at the muzzle". So that is what I am asking you all...

What do you think?,

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Unread postAuthor: D_Hall » Sat May 10, 2008 10:14 pm

I think this is what GGDT was written for.
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Unread postAuthor: psycix » Sun May 11, 2008 6:40 am

I dont think there is one and the same answer for all guns...
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Unread postAuthor: lockmanslammin » Sun May 11, 2008 12:54 pm

D_Hall, I hear you on that one. I'm going to model a ton of variations in GGDT and draw some two sided graphs of my cannon with variations of barrel length extensions and chamber lengths so I can visualize it better. What I'm planning on doing is to solder full port ball valves in the chamber so I can vary my C:B for different ammo, and different air sources. Right now I am at 23.231:8.398 or 2.76:1.

So my ratio isn't very efficient, But if I'm using a compressor I get a little more power. Unfortunately if I want to use my stirrup pump it takes forever to fill to high pressure. I found using GGDT that I can "shut off" a good chunk of my chamber and only lose 14fps with a marble at 400psi.

At first this started out as me trying to make the gun more accurate though so I would also like a setting that will dramatically reduce muzzle blast. Unfortunately looks like its a power accuracy trade off.

GGDT is a hell of a tool.

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Unread postAuthor: jimmy101 » Sun May 11, 2008 7:48 pm

lockmanslammin wrote:At first this started out as me trying to make the gun more accurate though so I would also like a setting that will dramatically reduce muzzle blast. Unfortunately looks like its a power accuracy trade off.

Crown the barrel, or if you are using a sabot or wading, check that it is consistent.

There is no fundamental reason why higher power should lead to less accuracy, indeed the opposite should be true.

So people say that a rule of thumb is to go between .5:1 and 4:1, what pressure are they at. A better way would be to say "what would be the best amount of PSI left in the gun when the projectile is at the muzzle". So that is what I am asking you all...

If someone quotes a CB range of 0.5 to 4 then they have pretty well covered the question of what pressure the gun is operating at. It doesn't matter what the gun is operating at, the quoted CB range is a factor of 8.

There is no such thing as the "best amount of PSI left in the gun". If you want to maximize efficency (which most pneumatic spudders don't really care about) then the gun should end up at just a little above atmospheric. If you want to maximize muzzle velocity then the gun should end up at whatever the max pressure of your compressor is, in other words, the chamber volume should be much bigger than the barrel volume.
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