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graphed GGDT results for small pneumatic

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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graphed GGDT results for small pneumatic

Unread postAuthor: lockmanslammin » Sat May 17, 2008 9:11 pm

Thought you guys might find this interesting or have some opinions. I'm trying to decide on a good C:B ratio, and I thought visualizing it might help, so I drew a graph. I'll put it up in jpeg and bitmap incase one is clearer than the other. Here you go:

<a href="http://s117.photobucket.com/albums/o75/lockmanslammin/?action=view&current=graphinjpeg.jpg" target="_blank"><img src="http://i117.photobucket.com/albums/o75/lockmanslammin/graphinjpeg.jpg" border="0" alt="jpeg graph"></a>

EDIT: I guess photobucket switches everything to jpeg so I erased the other pic....

What do you think?

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Unread postAuthor: POLAND_SPUD » Sat May 17, 2008 9:21 pm

hmmm I don't understand why 11 inch chamber results in higher MV than 12 inch chamber

anyway nice graph it shows quite well that increasing chamber size is pointles above some point
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Unread postAuthor: lockmanslammin » Sat May 17, 2008 9:26 pm

ya, I figured that was just a goof on GGDT's part but who knows... so do you think I should go for 11" chamber, and a 6:1 ratio?

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Unread postAuthor: POLAND_SPUD » Sat May 17, 2008 10:15 pm

nah... 6 inch chamber is the longest I would use.... but hey it's up to you.. it depends what you need etc.

if you want to build a plinker you should choose for example a 3 inch chamber as it can be easily pumped and it is still offers good velocity

if you want more velocity you will choose 12" chamber


first of all you have to decide what is important for you... keep in mind that 0.12g BB would loose its speed very fast so consider using heavier ammo ( 6mm ballbearings are 1g each) what is more, iI don't think you will notice the difference between 1140 fps and 1240 fps so it might be said that it doesnt make much sense to go for anything longer than 6" (but 3" is still quite good and you could pump it faster)
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Unread postAuthor: psycix » Sun May 18, 2008 7:19 am

A small chamber will do.
Note that your graph is somewhat misleading to the eye:
It starts at 1070 fps.
If you would make one starting with 0 fps, you would see that the difference between the c:b ratio is not very big.
20% more speed, for a 600% as big chamber....
I would go with a small chamber. 4"will certainly be more then enough.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 7:50 am

and it is much easier to pump up with a small chamber, yet deadly if you get hit in the eye
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Unread postAuthor: jimmy101 » Sun May 18, 2008 10:40 am

Like psycix said, very small increase in performance for a huge increase in chamber volume.

Instead of increasing the chamber you'll get a heck of a lot more "bang for your buck" if you add an inch to the barrel length.

Redo your analysis with a 13" barrel. I think you'll find that'll the performance increases a heck of a lot more with the slightly longer barrel than it does because of increasing the CB from say 3:1 to 6:1.
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Unread postAuthor: lockmanslammin » Sun May 18, 2008 11:06 am

Thanx for all the opinions guys. Seems everybody is in the same boat on this one. I was planning on highest velocity possible, but I think I will cut it back to 6 inches now.

Psycix, I wanted the graph to be very excentuated so the curve would be very easy to get an opinion on. I was kinda curious what it would look like though if I had started at 0" of chamber and 0fps.

I do have a large stirrup pump so any of the sizes would be relatively quick to fill up. But I suppose that, still, the quicker the better.

So what do you guys think a 1200 fps plastic bb will do?
Or a 6mm ball bearing for that matter which will end up going 917 fps?

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Unread postAuthor: POLAND_SPUD » Sun May 18, 2008 11:59 am

hey lock you ask very difficult questions... ekhm difficult to answer

if you can buy 6mm ballbearings use them... if you want to use 6mm barrel that's the best spherical projectile you can get....
if you want to get more muzzle energy form 6mm gun you should try shooting nails... just make sure that their nailheads are about 6mm in diam.
since nails are heavier than ballbearings you would loose some velocity but you'd get more muzzle energy

you can always use other calibres, just use GGDT to see how much muzzle energy you could get from different barrels and ammo

here is a table showing mass of chromed ballbearings in all sizes http://gewa.com.pl/page.php?d=kulki1wymiary (yeah it's in polish but you don't need to speak polish to read that table)

OMG sorry 6mm ballberaings are ~0.88 g each, not 1g
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Unread postAuthor: SpudFarm » Sun May 18, 2008 12:36 pm

you will be stuck at the sound barrier whatever you try if you use normal room temperature air.
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Unread postAuthor: lockmanslammin » Sun May 18, 2008 12:58 pm

Speed of sound being 1129fps? <a little wikipedia search...

Polund, I will probably be trying all sorts of ammo once the thing is built.
Maybe making some darts out of music wire or something. I would be afraid the nail head would be chewing the inside of my barrel up.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 1:03 pm

it wont destroy your barrel if you put a layer of tape on
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