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Need some quick advice before i get going today

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 some quick advice before i get going today

Unread postAuthor: mafia_mike » Mon Jun 23, 2008 11:03 am

Alright, I've gotten my first pneumatic cannon up and running, ive got my potatoes (other random ammo)...only problem though is that i think m y C:B ratio is way off. when i went to home depot i went ahead and bought a 10' x 2'' barrel, and i have a 10' x 3'' chamber. i would love to start shooting today, and i have a 3'' coupler ready to go if i need to do any cutting. im wondering, with my 10' barrel, would a 5' chamber be better then my 10' chamber? at first i had thought the 10' chamber would be much better because there would be a much higher volume of air to use, but after some searching in the forums, it seems like a good C:B ratio applies to all cannons weather it be combustion or pneumatic. for example, i just tested my cannon about 10 minutes ago at 100 psi. with a good seal in the barrel with a potato and the shot didn't seems as energetic as one would expect.

any suggestions would be appreciated, thanks!
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Unread postAuthor: Maniac » Mon Jun 23, 2008 11:17 am

Try to keep a ratio of 1:1 that "usually" will give you the best preformance. In other words keep the the volume the same or close to the same in the chamber and the barrel.
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Unread postAuthor: mafia_mike » Mon Jun 23, 2008 11:50 am

yes i think if i calculated correctly my C:B ratio is around 987,585:1 ... so thats why im thinking about chopping the chamber in half
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Unread postAuthor: igpay » Mon Jun 23, 2008 11:53 am

mafia_mike wrote:yes i think if i calculated correctly my C:B ratio is around 987,585:1 ... so thats why im thinking about chopping the chamber in half

Actually, I think its more like 2.25:1. Thats if your using the 10' 3" chamber and the 10' 2" barrel.
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Unread postAuthor: starman » Mon Jun 23, 2008 12:00 pm

mafia_mike wrote:yes i think if i calculated correctly my C:B ratio is around 987,585:1 ... so thats why im thinking about chopping the chamber in half


Yep cutting the chamber down to 5' would give you a little over 1.1:1...probably a little more practical size.
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Unread postAuthor: homedepotpro » Mon Jun 23, 2008 12:45 pm

c:b ratio's are mostly important with combustions. with a pneumatic, more air and/or higher pressure equates to more power. The larger the chamber (if pressure is held constant) will give you more even force on the ammo as the compressed air expands down the barrel.
btw since when does home depot sell 3'' pressure rate pvc. I hear their products differ a little bit in different locations but i've still never seen 3''
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Unread postAuthor: no-limit » Mon Jun 23, 2008 1:12 pm

He said he had the 10' by 3" already.
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Unread postAuthor: mafia_mike » Mon Jun 23, 2008 1:43 pm

lol, i dont think you caught the fact that i was WAY exaggerating when i said "987,585:1" ...im just about done solvent welding all my pieces into place and ill let it dry and hopefully i may be able to test fire it again soon.
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Unread postAuthor: Pilgrimman » Mon Jun 23, 2008 2:16 pm

Part of the problem may be your potato. 1 1/2"is perfect for potatos. 2" is pushing it. Other than that, it seems like a nice cannon! What kind of valve are you using?
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Unread postAuthor: mafia_mike » Mon Jun 23, 2008 2:42 pm

yea ive been messing around with the GGDT a lot and ive noticed (also with common sense physics) that the smaller/smaller more heavier object will travel further. i got the 2'' barrel because it seems to me that the average ordnance size is 2'' or less. plus if i plan on shooting golfballs or something, i can always use wadding. im not looking for MAXIMUM performance here, just building this to get good practice with working on pneumatic cannons. really i can say that this thing could possibly be a 1/3 scale of the real cannon i want to build one of these days

since its my first pneumatic, im using the plain ol' 1'' modded sprinkler valve
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Unread postAuthor: pizlo » Mon Jun 23, 2008 2:47 pm

If its a pnue, the higher tour chamber volume the better, you can't make it worse, just loner to fill. My impression was that 1.8/1 was the best for pnues, I think others are talking about combustions.
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Unread postAuthor: Carlman » Mon Jun 23, 2008 2:52 pm

for a gun of that size and with that longer of a barrel you may be better off with 1 well lubed 2" ball valve.
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Unread postAuthor: mafia_mike » Mon Jun 23, 2008 2:52 pm

yea well, the chamber's been cut in half...too late now :D .... im just waiting for the solvent to dry so i can test it
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