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barrel size

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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barrel size

Unread postAuthor: fir3dp » Wed Jun 30, 2010 12:26 pm

for a nail dart , would a 6mm barrel or a 15mm barrel be more powerful? both at the same psi and barrel to chamber ratio.


thanks :D
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Unread postAuthor: ramses » Wed Jun 30, 2010 12:37 pm

the 15mm barrel will probably be more powerful, at least for a given length

For the 6mm barrel to have the same volume as the 15mm barrel, the 6mm one would have to be 6.25 times longer. I doubt you want that.

My advice also assumes you use a large enough valve in either case. if you use a 6mm porting valve for a 15mm barrel, performance will kind of suck.
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Last edited by ramses on Wed Jun 30, 2010 12:42 pm, edited 1 time in total.
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Unread postAuthor: Ragnarok » Wed Jun 30, 2010 12:38 pm

Depends on exactly how you're defining things, but energy is force x distance. And force (from pressure) is pressure x area.

A 15mm barrel has a considerably larger cross sectional area than a 6mm one.

If you've got barrels of the same length, 15mm therefore wins out.

However, barrels of the same volume would be a different matter (not including valve and flow losses, performance would be the same).
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Last edited by Ragnarok on Wed Jun 30, 2010 12:40 pm, edited 2 times in total.
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Unread postAuthor: metalmeltr » Wed Jun 30, 2010 12:39 pm

15mm is more powerful because it is aplying more force to the back of the round

for example 100psi over 1 square inch would be 100 pounds of force
100psi over 2 square inches would be 200 pounds of force
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Unread postAuthor: sgort87 » Wed Jun 30, 2010 3:01 pm

Nope. Did you hear what Rag already said? Energy = Force x Distance.

Let's say we use 100 PSI for simplicity's sake, and keep things in terms of pi. And Ramses already gave us the barrel volume ratio of 6.25 - we'll be using that too...

(r^2)pi x psi = F
F x D = E

(7.5^2)pi x 100 = 5625pi
5625 x 1 = E = 5625pi

(3^2)pi x 100 = F = 900pi
900pi x 6.25 = E = What? 5625 again? No way!

The barrels having the same volume will transfer equal energies. Your gun is as powerful as your chamber. Barrels can be adjusted for the projectile, and will add up to the same energy every time if adjusted right. (Give or take friction and resistances and all that hippie unicorn make-believe nonsense.)

The only thing that will make any difference is that the 15mm projectile will tend to be a bit more immune to wind stuff (That's right, wind stuff. It's a technical term.), and should maintain its speed better. Of course, this depends on mass too... If the 15mm ones are less dense than the 6mm, they'll probably do some crazier things in the air.

If it were me, I'd go with whichever round is more dense, and if they are the same, go with the 15mm.
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Re: barrel size

Unread postAuthor: boyntonstu » Wed Jun 30, 2010 3:06 pm

fir3dp wrote:for a nail dart , would a 6mm barrel or a 15mm barrel be more powerful? both at the same psi and barrel to chamber ratio.


thanks :D


I shoot a thin 10d nail in a 3/4" cork sabot at 900 fps using 600 psi.

The larger the barrel the more the oomph!
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Re: barrel size

Unread postAuthor: jackssmirkingrevenge » Wed Jun 30, 2010 5:16 pm

boyntonstu wrote:The larger the barrel the more the oomph!


...provided the valve is similarly scaled up...
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Re: barrel size

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Unread postAuthor: Technician1002 » Wed Jun 30, 2010 6:27 pm

When testing barrels on a 2 inch valve (fixed chamber and valve size) in barrel acceleration of rolled up t shirts was tested on 2.5, 3, and 4 inch barrels. A 3 inch barrel on a 2 inch valve provided better muzzle velocity. The increase was marginal, but the shorter denser shirt projectile flew much further. Shot fat rolled shirts fly much better then long skinny sausage shapes. Long shapes tend to tumble and lose speed much faster than shorter projectiles.

A 3 inch valve on a 3 inch barrel would have been a real performer.

I got to get to work and finish my 2.5 inch valve. A 4 inch valve is planned.
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Unread postAuthor: ramses » Wed Jun 30, 2010 9:20 pm

The main reason I suggested 15mm is that a 200ml 6mm barrel would be around 7 meters long, flow and frictional losses would become significant, and 7 meters is rather long for a barrel.

A 15mm barrel with the same volume would be ~1.13 meters, much more manageable.

and 200mL is considered fairly small.
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Unread postAuthor: fir3dp » Wed Jun 30, 2010 10:48 pm

my valve is 15mm btw

oooh i always thought that by fitting a small barrel on a huge valve and chamber would be more powerful. 6mm darts are much easier to make too. ok i shall switch to the 15mm one for darts then. although i had fun shooting BBs.

thanks guys :D
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