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Can someone direct me to the post on barell vs chamber 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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Can someone direct me to the post on barell vs chamber ratio

Unread postAuthor: Shango » Sat Jun 14, 2008 4:08 pm

I found it the other day but before I could bookmark it, I was distracted (wife nagging about my f****n' Spud Guns, if you can imagine!). I can't find it again.

I downloaded the GGDT but can't get it to open, Any ideas?

By way of an explaination:

I have two PVC chambers, 2 feet long x 4 inches in diameter, each with a 1 inch modified valve venting into a golf ball barrel.

I am trying to determine the optimum barrel length.
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Unread postAuthor: THUNDERLORD » Sat Jun 14, 2008 5:18 pm

I'm on a borrowed computer now.
So I don't want to download it but you could try again :
<a href="http://thehalls-in-bfe.com/GGDT/">Hall's GGDT</a>

It's a simple math formula for calculating cylinder volume of the chamber(s) and divide that by barrel (cylinder) volume.

For pneumatic I like about 1.25:1 or atleast 1:1.
For combustion it's said .8: 1 is best.

Other than that, hope someone else who's got it handy can help you out. :( 8) [/url]

EDIT: or you could try my old way: fill the chamber with fluid and pour it into the pipe you plan to use for barrel (sometimes faster/ more macho)!
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Unread postAuthor: ajb12345 » Sat Jun 14, 2008 5:22 pm

go to wiki. the ggdt page has some links
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Unread postAuthor: DYI » Sat Jun 14, 2008 8:26 pm

For pneumatic I like about 1.25:1 or atleast 1:1.
For combustion it's said .8: 1 is best.


If you're not willing to hand out proper information, please don't hand out any at all.

Assuming a pressure of 150 psi, with a fixed chamber volume, performance will hit the efficiency (and therefore power) sweetspot right around a C:B of roughly 0.2:1. This obviously varies slightly depending on other conditions like projectile mass (which suprisingly has a very large effect on optimal C:B), but it's a good place to start if you don't have GGDT.
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Unread postAuthor: Shango » Sat Jun 14, 2008 9:05 pm

Thunderlord,

I like your MACHO solution: "For pneumatic I like about 1.25:1 or at least 1:1."

Now, I'm a little confused (as Barbie says, "Math is hard!"). Which way is the ratio, the barrel contains 1.25 times the chamber, or vice versa

Appreciate your help :P
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Unread postAuthor: DYI » Sat Jun 14, 2008 9:20 pm

I like your MACHO solution: "For pneumatic I like about 1.25:1 or at least 1:1."


I can't really see where that's coming from... In your case, 0.2:1 will yield more than twice the muzzle energy of 1:1.

And, as the term C:B suggests, the ratio is Chamber:Barrel.
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Unread postAuthor: THUNDERLORD » Sat Jun 14, 2008 9:53 pm

DYI wrote:
For pneumatic I like about 1.25:1 or atleast 1:1.
For combustion it's said .8: 1 is best.


If your not willing to hand out proper information, please don't hand out any at all.

Assuming a pressure of 150 psi, with a fixed chamber volume, performance will hit the efficiency (and therefore power) sweetspot right around a C:B of roughly 0.2:1. This obviously varies slightly depending on other conditions like projectile mass (which suprisingly has a very large effect on optimal C:B), but it's a good place to start if you don't have GGDT.


I find your statements rediculous.
For one thing your "handing out proper information" statement.

It is proper information .I Do Like 1.25 cubic inches (or cubic centimeters) volume chamber for each 1 cubic inch (or cubic centimeter) of barrel volume.

You are the one who's given no helpful information and I don't care to argue but you make no sense. maybe if you write back and explain it will
somehow(?) :roll:
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Unread postAuthor: DYI » Sat Jun 14, 2008 10:27 pm

Okay, you've got a point there. Try to hand out useful information. He was asking what the optimum barrel length for his chamber was, not what C:B ratio you prefer to use.

I already gave out the useful information of the most efficient C:B ratio at 150psi. And as to the actual question: the golf ball barrel should be about 25' long for optimum performance, taking into account the low mass of a golfball.
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Unread postAuthor: THUNDERLORD » Sun Jun 15, 2008 12:42 am

Thankyou DYI,
It seemed like what I wrote had been taken out of context.
But it was already maybe misinterpreted a little.

In my reply I explained I didn't have D_HALL's GGDT handy.
That I hoped someone did.
And gave a link to it.
(seemed like no one would reply and a lot of activity at the time?)

As far as optimal barrel length for PVC type pressure,
I like 1.25: 1 only for portability/ wieldability.
I like 1: 1 for larger bore for the same reason.
Also I like to shoot heavier projectiles (sometimes longer than the barrel is).

I will have access to GGDT again soon.
I probably could've explained better with less information. 8)
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Unread postAuthor: jr » Sun Jun 15, 2008 1:41 am

shango: welcome

with that kind of set up you are looking at a mass of launch

I have one that is similar. mine is 2: 3" elbows, 2: 3" x 36" pipes, and 2: 1" modded sprinks. coupled to a 2" T with interchangable barrels. My gb is only 4' and the ball goes a good 300 yards.

You can get all technical or just have fun. As long as it goes really far really fast who cares! :twisted:
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Unread postAuthor: DYI » Sun Jun 15, 2008 9:41 am

Contrary to some of the information I've seen, using heavier projectiles actually increases the optimal barrel length for a given chamber volume and pressure.
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Unread postAuthor: no-limit » Sun Jun 15, 2008 12:40 pm

So what's the better C:B ratio for power, the 1.25:1 or the 2:1. Shango and jr do you guys have any pics?
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Unread postAuthor: Shango » Sun Jun 15, 2008 12:43 pm

No pics yet, I'm still in the procurement phase.
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Unread postAuthor: D_Hall » Sun Jun 15, 2008 1:11 pm

DYI wrote:Contrary to some of the information I've seen, using heavier projectiles actually increases the optimal barrel length for a given chamber volume and pressure.

This is due to most folks using undersized valves. Once flow in the valve chokes, projectile residence time becomes a key factor. The longer the time, the more air flows into the barrel, the more the gun can make use of the available energy stored in the chamber. Larger projectiles are slower, therefore their residence times are longer. Voila, more effective use of the stored energy.
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Unread postAuthor: jimmy101 » Mon Jun 16, 2008 5:05 pm

DYI wrote:Contrary to some of the information I've seen, using heavier projectiles actually increases the optimal barrel length for a given chamber volume and pressure.

Heavier projectiles also decrease the importance of a fast opening valve. With very massive projectiles any decent valve with the same Cd gives the same performance regardless of opening time (as long as the opening time is not extremely slow).
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