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is my barrel to chamber ratio good enough?

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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is my barrel to chamber ratio good enough?

Unread postAuthor: paintballuh » Fri Jul 13, 2007 6:41 pm

i built my first pneumatic [im wating for the cement to cure], and im wondering if a 1:3 ratio will work well enough. [note, the barrel is the 1, the chamber is the 3]


and yes, i calculated the volumes and simplified the ratio.


UPDATE: this gun is now scrapped. i have built a new successful one...WITH primer.
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Last edited by paintballuh on Sun Jul 15, 2007 10:48 pm, edited 1 time in total.
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Unread postAuthor: singularity » Fri Jul 13, 2007 6:55 pm

well what are you shooting, if its really light, like a t-shirt or something that will probably be fine. but if you plan on shotting lead slugs there gonna go no where. any ways you tend to want the chamber to have more volume than the barrel (pretty obvious) until the ratio is about 4:1, at which point a larger chamber only barely increases performance. little more info would be helpful, like the ammo, the desired range, and the valve
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Unread postAuthor: paintballuh » Fri Jul 13, 2007 7:21 pm

well, i plan on using it as an airsoft shotgun, as good ones run for about $500, i found it much easier to resort to a small pneumatic. the barrel is 7inches of 3/4 PVC, the chamber is 21-22 inches, including fittings. the valve is just a simple ball valve...heres a pic.

Image
[AA battery show to represent size]

the duct tape peeking out of the endcap is to make it tighter, the end cap was just a bit loose, i put pvc cement under the tape, and on the outside.
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Unread postAuthor: MrCrowley » Fri Jul 13, 2007 7:26 pm

Its called PVC cement for a reason, its for PVC on PVC bonds, not PVC on duct tape. Also its a pneumatic you need to be using primer with it.
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Unread postAuthor: paintballuh » Fri Jul 13, 2007 7:31 pm

you must remeber, this a n00bmatic cannon, cut me some slack.
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Unread postAuthor: singularity » Fri Jul 13, 2007 8:01 pm

yea the ratio is fine for what your doing and most of the time it is referred to as a chamber to barrel ratio. if you wanted to get better performance go fore a 3/4 modified sprinkler valve it will open much faster than a ball valve. that was you can have a real trigger (well a blow gun valve close enough to a trigger)
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Unread postAuthor: paintballuh » Fri Jul 13, 2007 8:13 pm

i might do that on my next pneumatic. i just got done regluing the thing, i decided to heed MrCrowley's word. i removed all the tape and used lots of cement. i dont have any primer :'( and im too lazy to go get some.
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Unread postAuthor: noname » Fri Jul 13, 2007 8:17 pm

Then we'll all laugh and say "What a dumbassed noob" when the piece of crap blows up.
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Unread postAuthor: spudgunnerwryyyyy » Sun Jul 15, 2007 6:00 pm

Hey noname, 2 of my guns have no primer and they have gone up to 125 psi no problem chill out man.[/quote]
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Unread postAuthor: homedepotpro » Sun Jul 15, 2007 6:23 pm

spudgunnerwryyyyy wrote:Hey noname, 2 of my guns have no primer and they have gone up to 125 psi no problem chill out man.
[/quote]

people drive drunk all the time too. that doesn't make it ok
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Unread postAuthor: f.c » Sun Jul 15, 2007 6:33 pm

in Australia we have this special stuff which is pvc pressure glue its green and is able to be used without primer. as long as you do the plumbers trick.
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Unread postAuthor: Marco321 » Sun Jul 15, 2007 7:25 pm

Hey

in Australia we have this special stuff which is pvc pressure glue its green and is able to be used without primer. as long as you do the plumbers trick.


I'm in Australia aswel and i use that glue, on the back it does say to use primer, i must admit i didn't use primer until i decided to rebuild my gun. The glue i use does melt the plastic a bit, but not like primer does.

What is the plumbers trick you speak of?

Now back on topic. 1:3 should be fine if that is B:C ratio, if that is your C:B ratio it wont be very powerful at all. Usually you speak of the ratio in terms of Camber : Barrel not Barrel : Chamber, to find C:B, divide the chamber by the barrel and that will be your ratio.
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Unread postAuthor: Modderxtrordanare » Sun Jul 15, 2007 8:47 pm

I didn't think C:B ratios had a noticable effect on pneumatics.. :?:
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Unread postAuthor: Marco321 » Sun Jul 15, 2007 9:50 pm

Modderxtrordanare wrote:I didn't think C:B ratios had a noticable effect on pneumatics.. :?:


It does, think about it. The air has to expand, it will expand until it reaches the surrounding air pressure right, and the air pressure is stored as potential energy in the chamber. The only place the air can escape and spread out to reach its surrounding air pressure is by pushing air molecules down the barrel, creating kinetic energy, which gets transferred to the projectile, giving it motion. The more air stored in the chamber at a set pressure, the longer it takes to reach the pressure of the environment, which means the same kinetic energy will be applied to the object fore longer. Since air expands at around the speed of sound and wind resistance and friction are working against the projectile, the longer the kinetic energy is applied to the projectile the faster it goes.

In conclusion if the air pressure reaches the surrounding pressure while the projectile is halfway down the barrel, the projectile will begin to decelerate because of friction and air resistance, this also means the projectile will only reach a certain speed because of friction and air pressure. If the air reaches the surrounding pressure, when the projectile has left the barrel, the projectile will have been accelerated all the way down the barrel, giving it a higher velocity.

I hope that made sense to everyone lol.
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Unread postAuthor: noname » Sun Jul 15, 2007 10:12 pm

Or you can put it this way: a 1" chamber won't give you good performance with a 2" barrel twice the length.
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