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Pneumatic C:B ratio revisited

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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Pneumatic C:B ratio revisited

Unread postAuthor: DYI » Mon Jul 23, 2007 2:28 pm

I've always heard that a good C:B ratio for a pneumatic is 1.5 or 2:1, and that after about 4:1, increasing chamber size isn't really going to increase performance.
Recently I have been playing around with GGDT to try and optimize performance of my burst disk gun (the SCTBDC-100), after Clide advised that I use a 4' barrel rather than the 2' which gave it a ~2.5:1 C:B ratio. I decided to see at what point increasing barrel size won't help any more, and was rather surprised that at 160psi I could use a 10' barrel and get about 1030 foot pounds of muzzle energy (almost 400 more than with a 4' barrel). This would give a C:B ratio of about 0.5:1. If I added another 2' to the barrel, their would still be a noticeable increase in velocity (and therefore an increase in muzzle energy), although 10' is the longest length that I can get of ABS pipe, and I'm not dumping $70 on a 12' length of 1.5" copper just to get an extra 50 foot pounds of energy.
Now I will get to the point; Why has the 2:1 ratio for pneumatics been so widely propagated? To me it seems to be kind of arbitrary, and 1:1 would usually be better, even at relatively low pressures (like, 80psi or so). As pressure increases the ideal ratio changes drastically. At 300 psi GGDT shows noticeable increases in performance even as the barrel length passes 240 inches (a .25:1 C:B ratio). It seems as though some high pressure launchers could definitely benefit from this.
In conclusion, the ideal C:B ratios for pneumatic cannons seem to vary just as much as those of hybrids, depending on the pressure used. GGDT is really useful for these sort of things, but for the people that can't, or don't, use it, this info could probably be useful.
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Unread postAuthor: SpudBlaster15 » Mon Jul 23, 2007 2:46 pm

The reason for which people use large C:B ratios is that they are building a chamber around a barrel. Unlike in the case of a combustion launcher where an oversized chamber decreases performance due to slower combustion time, and thus slower pressure increase and lower initial acceleration, an oversized chamber will increase the performance of a pneumatic launcher by creating more overall acceleration throughout the barrel. A large quantity of energy will be wasted, but acceleration will increase throughout the entire barrel.

However, if you are designing a barrel around a chamber, you will achieve the best performance when the barrel length allows for zero acceleration at the muzzle, meaning all of the chamber energy is transferred to the projectile and none is wasted, and this ratio depends upon the pressure used. For a pneumatic launcher using an average pressure level, around 100psi, the optimal ratio is around 0.25-0.3:1.
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Unread postAuthor: boilingleadbath » Mon Jul 23, 2007 3:51 pm

Spudblaster sums up the situation pretty well; people use high ratios because it gives them more performance for the overall size of the launcher.

(incidentally, they sell these things called "couplers"... you might be interested in them, especially if you wish to build impressively long barrels)
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Unread postAuthor: DYI » Mon Jul 23, 2007 4:31 pm

Hmm... I've never heard of these "couplers" before :lol:
I don't think that a barrel longer than 12' would really be worth the trouble (unless you own FEAR or the SWAT gun).
I seem to have missed my target audience with this; I've attracted the intelligent people who already know what they're doing, not the beginners who would actually benefit from what I said. I know that a lot of people build launchers with convenience in mind, but you have to admit that there are more posts telling noobs that the optimal ratio is 2:1 than there are saying that a convenient ratio is 2:1.
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Unread postAuthor: CpTn_lAw » Mon Jul 23, 2007 5:22 pm

This thread has no reason to exist.You are telling things we already know. Every penumatic with a long barrel will have a longer acceleration and thus a high muzzle energy...
This is right until there is at least 0-1 psi when the projectile comes out of the barrel. you could have a ridiculously small chamber pumped at 300 bars and still get more power out of it than your friend who would have a 6" by 4' chamber at 100 psi...
There are two things very important when building a pneumatic cannon (and i know what i'm talking about i've built about 15 pneumatics in the last 4 years) :
1) Initial acceleration. The higher the pressure, the bigger the boost.
2) Momentum of acceleration. The longer the boost , the higher the muzzle energy.

Everything is a matter of average. It would be ridiculous to say your 30 meters cannon can throw a bowling ball 40 seconds into the skys... i mean come on we're not at pumpkin chunkin'...

A ratio alone is nothing, it's a proportionality number in the vaccum. You need to consider the overall length of the gun, the barrel length and the pressure of the thing... If you're using abs, then you definately won't be able to pump your thing at 300 psi unless you want to be called "scarface". So in that case, a long barrel, a big chamber and small pressure. We ain't talking about efficiency, we're talking about power. Sure if you want efficiency, your neighbors won't hear a damn shot.

Now it's not that serious to loose energy in the air...A good ratio is a ratio that fits your requirements, (materials, pressure, cost) and that provides enough power. So i would be tempted to say that there is no "optimum ratio" for pneumatics. Only things that provide noise, power, or both, or none.
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Unread postAuthor: Pilgrimman » Mon Jul 23, 2007 5:35 pm

I know it's not safe to use ABS in pneumatics, but I've heard that it only rips upon failure. Would the fittings shatter though? If not, I don't understand how it could be that dangerous, other than hearing loss at high pressures...
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Unread postAuthor: CpTn_lAw » Mon Jul 23, 2007 6:10 pm

Other than going at the hospital for a "plastic shrapnel removal"...my personnal experience tells me that even at 60 psi, a failure is painfull. :)
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Unread postAuthor: DYI » Mon Jul 23, 2007 6:42 pm

The BARREL is ABS, the CHAMBER is galvanised steel and brass.
Law: Considering that you don't think this thread should exist, you certainly wrote a long post about it. You are doing exactly what I did, stating info that most of us already know. I didn't exactly say that I had made a revolutionary discovery or anything. Yes, ABS does rip upon failure, which makes it a good chamber material in combustions, and a decent barrel material in anything except high mix hybrids. It doesn't take very high chamber pressures to cause hearing loss in the event of a catastrophic failure of the chamber material.
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Unread postAuthor: CpTn_lAw » Mon Jul 23, 2007 7:08 pm

Well, sorry if i may have sounded em...what's the word...f***, if i sounded agressive, this wasn't meant to be this way. What i truly wanted to say is that if people can't think about that by themselves(i mean the whole C:B thing) , then they shouldn't build pneumatics...that's all!
Sorry for the agressive looking post! you know i'm not like that !!! :D
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Unread postAuthor: DYI » Mon Jul 23, 2007 7:23 pm

It's all good 8)
In hindsight, this topic probably wasn't necessary, but at least one person who reads this will probably learn something from it...
On a side note, at what point do you think a pneumatic makes the transition from potato gun to punkin' chucker? I wonder what the SWAT gun would be?
EDIT: Just reread one of you earlier posts, if you are working with a given chamber size, the most efficient ratio IS the most powerful.
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Unread postAuthor: mtronic » Mon Jul 23, 2007 7:51 pm

umm CpTn_lAw just a quick thought, and I'm not trying to be rude but don't gamo shadow 1000's have a muzzle velocity of between 900-1000 fps? Hence the name 1000. I know gamo likes to over exagerate a bit but most tests come out at about 950fps. I'm no expert on the matter though but to the best of my knowledge unless there is something drastically wrong with the rifle it should be sitting close to 1000fps.

Ps thanks for the useful C:B info, I've recently built my 1st pneumatic and I love it. ATM I use a 2ltr coke bottle but will be upgrading to a better chamber and the info You have given is very helpful :)
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Unread postAuthor: CpTn_lAw » Tue Jul 24, 2007 4:30 am

Mtronic you are absolutely right! But...in france, we cznnot pocess weapons that developp more than 10 joules if we don't have a permit, or a shooting license, which i don't have. So gamo sells 10 joules rifles for people who just want to plink around like me lol! But don't worry, with the dieseling and light weight of the pellets i use, i must shoot at nearly 800 fps.


at DYI , I don't know at what point a cannon is considered a chuker....maybe when it shoots a pineapple at 3/4 mile...and uses more than basic 100 psi.
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Unread postAuthor: jackssmirkingrevenge » Tue Jul 24, 2007 5:06 am

mtronic wrote:umm CpTn_lAw just a quick thought, and I'm not trying to be rude but don't gamo shadow 1000's have a muzzle velocity of between 900-1000 fps? Hence the name 1000. I know gamo likes to over exagerate a bit but most tests come out at about 950fps. I'm no expert on the matter though but to the best of my knowledge unless there is something drastically wrong with the rifle it should be sitting close to 1000fps.


I think you'll find that the actual velocities are below 900 fps - disregard any advertising for airguns that only quotes the velocity of the pellet. Velocity figures without pellet weight are meaningless, what you should check out is the ft/lb rating - most advertisers intentionally omit this specification as it reveals that the high velocity they quote is with uselessly lightweight pellets.
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Unread postAuthor: pinkham21 » Tue Jul 24, 2007 8:27 am

jackssmirkingrevenge is right. Those rating are for light weight pellets that transfer little muzzle energy. If you increase the weight your projectile your velocity slows down, but the up side is the increase of muzzle energy transfered to target. In other words you will do more damage with the heaver project than the light, but you sacrifice velocity.
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Unread postAuthor: DYI » Tue Jul 24, 2007 2:54 pm

Does anyone know the conversion between ft/lb and joules? I couldn't seem to find it. Firing uselessly light projectiles is a good way to make performance figures look better. For example, I have a cannon that can fire at >1100fps (using airsoft rounds), but they can't even go through 1/2" plywood, so what's the fun in that?
A cannon that can fire a pineapple 3/4 of a mile...
I wonder what the muzzle energy of that would be?
Any idea what pressure a punkin' chunker usually operates at?
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