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Pneumatic Air Cannon C:B 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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Unread postAuthor: Ragnarok » Mon Feb 23, 2009 4:49 am

Lokoyo wrote:I rather not use a sabot because I might use this cannon in a water war.

With the right kind of sabot, that shouldn't be a problem. You shouldn't really engage at "point blank" anyway, and with a light enough sabot, it'll slow to a halt due to drag long before it ever gets close to a target. (It should also be light enough that should it hit someone the results are laughable).

Things like plastic drinking cups are popular in the right barrel sizes - unlikely to cause injury, very quick to slow down from drag, etc, etc.

Alternatively you can just use lower pressures and make do with a low velocity cannon.
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Unread postAuthor: trigun » Mon Feb 23, 2009 5:36 pm

Well if he is truly worried about the sabot tie a string to it and attach it at the end of the barrel. That would keep it safe and reusable.
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Unread postAuthor: Lokoyo » Mon Feb 23, 2009 5:55 pm

ramses wrote:FYI: for our purposes water is not compressible.

but yeah, water gets heavy. 2"x4' of water weighs nearly 5.5 lbs. that's a column of water, if you only used a foot, recoil would be much reduced. some sort of plunger might be required to expel the water effectively and not just "blow bubbles" through the column of water that is your barrel. big bubbles.

I know I saw somewhere that someone's launcher broke because of recoil. It might have been the archives. it's only really an issue in an over-under.

If you go that route(over-under), anchor the barrel to the chamber with pieces of plywood/whatever in between the barrel and chamber, and use pipe clamps to hold them securely together. brace it against the ground so as not to hurt yourself.


If you intend to use water balloons, use a slow or bad flow valve and a longer barrel to avoid smacking it with air and maintain respectable performance. also try regular balloons, they aren't made to burst easily.


Good point about the bubbles, never thought of that.
I guess i'll test with the 2" by 4' and move down until I find the right one.

What do you mean the cannon breaks? How so?
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Unread postAuthor: Hotwired » Mon Feb 23, 2009 8:48 pm

The barrel can act as a large lever on the fittings making up the 180* turn from chamber to barrel. Which isn't a good thing, so if it's any notable length it's best to have barrel and chamber secured to each other.

On the main topic:

Pneumatics don't have C:B ratios.

Yes you can compare the volume of each but it's pretty much irrelevant as pneumatics can use a vast range of pressures.

A 4:1 pneumatic using 20psi air is wildly different to a 1:30 pneumatic using 2500psi Helium.

Not even usable on the same target range.

C:B ratios only make sense for Combustions because if it's too low a vacuum can be formed behind the projectile before it can leave the barrel which naturally knocks performance. To high and it increases firing noise.

Don't go after a ratio pulled out of the air for the sake of it, fiddle about in GGDT until you get the power from the chamber size you're comfortable with, the pressure you can get and the practical length of barrel you can use.
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Unread postAuthor: dudeman508 » Mon Feb 23, 2009 11:02 pm

Lokoyo wrote:I rather not use a sabot because I might use this cannon in a water war.


I dont mean full on sabou I meant a little bit of tolet paper.
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Unread postAuthor: Ragnarok » Tue Feb 24, 2009 3:43 am

dudeman508 wrote:I dont mean full on sabou I meant a little bit of tolet paper.

In that case, a better term would be wadding.
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Unread postAuthor: Lokoyo » Tue Feb 24, 2009 5:57 pm

I will upload my design I have (It's really basic) soon once I get everything how I like it
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Unread postAuthor: jimmy101 » Wed Feb 25, 2009 2:16 pm

Common hotwired, you know better than that.

CB ratio in a combustion gun has nothing to do with "suck-back". The "suck-back" ratio is vastly different than the optimum CB for efficiency. Efficiency is how "optimal" CB is defined in a combustion gun. The most powerful CB for a combustion gun is not the same as the "optimal" CB.

A pneumatic most certainly does have a CB ratio. And not just because you can calculate the chamber to barrel ratio. It is because the CB ratio defines the ratio of the pressure before firing with the pressure when the round just exits the barrel. For any gun being fired with a starting pressure significantly above 1 ATM the CB ratio gives a good estimate of the energy you'll get out of the gun, as well as the (generally no all that important) efficiency of the gun.

Yes, as you change the starting pressure the performance of a gun will change, but the CB ratio still provides information on how the same barrel and ammo will perform as a function of the chamber, or the same chamber and ammo as a functon of the barrel length.

Using the CB ratio of 4:1. The pressure at ammo exit will have dropped by 20%. The pressure (hence force) during ammo transit will go from the intitial chamber pressure to 80% of that pressure.

Using a CB of 2:1. The pressure at ammo exit will have dropped by 33%. The pressure during ammo transit will go from the intitial chamber pressure to 67% of that pressure.

Using a CB of 1:1. The pressure at ammo exit will have dropped by 50%. The pressure (hence force) during ammo transit will go from the intitial chamber pressure to 50% of that pressure.

At a huge CB of 100:1 the pressure pushing the ammo doesn't change (within experimental error) as the ammo transits the barrel. With a large CB you are getting as much energy into the projectile as possible given the starting pressure.

With a low CB you are getting as much energy into the ammo as possible figuring in the "cost" of pressurizing the chamber. The "cost" includes pumping cost (insignificant with a shop compressor, very significant with a hand pump), chamber cost (bigger=more $), ergonomics (do you really want a 10' long chamber on a 2' barrel?), esthetics etc.
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Unread postAuthor: Lokoyo » Thu Feb 26, 2009 7:33 pm

jimmy101 wrote:Common hotwired, you know better than that.

CB ratio in a combustion gun has nothing to do with "suck-back". The "suck-back" ratio is vastly different than the optimum CB for efficiency. Efficiency is how "optimal" CB is defined in a combustion gun. The most powerful CB for a combustion gun is not the same as the "optimal" CB.

A pneumatic most certainly does have a CB ratio. And not just because you can calculate the chamber to barrel ratio. It is because the CB ratio defines the ratio of the pressure before firing with the pressure when the round just exits the barrel. For any gun being fired with a starting pressure significantly above 1 ATM the CB ratio gives a good estimate of the energy you'll get out of the gun, as well as the (generally no all that important) efficiency of the gun.

Yes, as you change the starting pressure the performance of a gun will change, but the CB ratio still provides information on how the same barrel and ammo will perform as a function of the chamber, or the same chamber and ammo as a functon of the barrel length.

Using the CB ratio of 4:1. The pressure at ammo exit will have dropped by 20%. The pressure (hence force) during ammo transit will go from the intitial chamber pressure to 80% of that pressure.

Using a CB of 2:1. The pressure at ammo exit will have dropped by 33%. The pressure during ammo transit will go from the intitial chamber pressure to 67% of that pressure.

Using a CB of 1:1. The pressure at ammo exit will have dropped by 50%. The pressure (hence force) during ammo transit will go from the intitial chamber pressure to 50% of that pressure.

At a huge CB of 100:1 the pressure pushing the ammo doesn't change (within experimental error) as the ammo transits the barrel. With a large CB you are getting as much energy into the projectile as possible given the starting pressure.

With a low CB you are getting as much energy into the ammo as possible figuring in the "cost" of pressurizing the chamber. The "cost" includes pumping cost (insignificant with a shop compressor, very significant with a hand pump), chamber cost (bigger=more $), ergonomics (do you really want a 10' long chamber on a 2' barrel?), esthetics etc.


Thank you very much. That is really going to help :D
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Unread postAuthor: dudeman508 » Fri Feb 27, 2009 7:37 pm

Ragnarok wrote:
dudeman508 wrote:I dont mean full on sabou I meant a little bit of tolet paper.

In that case, a better term would be wadding.


I dont think so because i would wrap the paper towel around the water balloon so their isn't any friction that might pop it before it leaves the barrel.
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Unread postAuthor: inonickname » Sat Feb 28, 2009 3:26 am

The problem with shooting water balloons is acceleration. Some guy tested a ballvalve vs. a barrel sealer and he found that the ballvalve rarely damaged balloons, but the piston valve consistently popped them before leaving the barrel. The gentler acceleration of the ballvalve would have yielded less distant but popped them much less.
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Unread postAuthor: Hotwired » Sat Feb 28, 2009 7:16 am

@ jimmy, I seem to have missed what I meant but meh, it's not important.

As for the water balloons, the acceleration won't destroy them but the friction between the rubber and the barrel will tear them up if they're being pushed out with enough force. If you put a layer of material between the balloon and barrel you can use any pressure.
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Unread postAuthor: inonickname » Sat Feb 28, 2009 8:15 am

Hotwired wrote:@ jimmy, I seem to have missed what I meant but meh, it's not important.

As for the water balloons, the acceleration won't destroy them but the friction between the rubber and the barrel will tear them up if they're being pushed out with enough force. If you put a layer of material between the balloon and barrel you can use any pressure.


Alright, I suppose this would be a better way to put it:

Drag your knuckles along a piece of carpet slowly, gradually gaining speed.

Now drag your knuckles along carpet as fast as you can straight away.

The first one hurts (and damages) less doesn't it. That's closer to a ball valve. The second will hurt a lot more and that's closer to a piston valve. Simplest way I can put it.

But yes, a light lubricant or a wad or sabot will help the matter.
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Unread postAuthor: dudeman508 » Sat Feb 28, 2009 8:34 am

dudeman508 wrote:
Ragnarok wrote:
dudeman508 wrote:I dont mean full on sabou I meant a little bit of tolet paper.

In that case, a better term would be wadding.


I dont think so because i would wrap the paper towel around the water balloon so their isn't any friction that might pop it before it leaves the barrel.


If you did it well you wouldnt have to worry about acceleration at all.
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Unread postAuthor: inonickname » Sat Feb 28, 2009 8:51 am

dudeman508 wrote:
dudeman508 wrote:
Ragnarok wrote:
dudeman508 wrote:I dont mean full on sabou I meant a little bit of tolet paper.

In that case, a better term would be wadding.


I dont think so because i would wrap the paper towel around the water balloon so their isn't any friction that might pop it before it leaves the barrel.


If you did it well you wouldnt have to worry about acceleration at all.


Wrong, if you have enough of an acceleration force, and the balloon is a loose enough fit it will become deformed and possibly pop. It's like getting a water balloon, and squeezing it in your hands. It pops, doesn't it?

Therefore it would be good to make the balloon a good enough fit in the sabot so it can't expand further than the rubber can manage, but can still discard the sabot. There's where your challenge is.
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