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Question about barrel length

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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Question about barrel length

Unread postAuthor: Extrusion » Sat Oct 21, 2006 12:29 pm

i made my first over under pneumatic a few days ago and i must say it it awsome but i think the barell may be a tad to long the chamber is 28" long and 2" wide the barrel i have been using is 54 inches of 1/2" solid black iron...very heavy not i do have a peice of 30" long black iron barrel wich is much much lighter and i want to use it but wich would be better for all around performance and power the 54" long barrel or the 30" long i tried to ggdt my gun but i couldnt figure out the valve and all and it said "Barrel choking flow" and "Barrel unecicarily long" what does this mean?
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Unread postAuthor: joannaardway » Sat Oct 21, 2006 1:24 pm

When GGDT says "Barrel excessively long" it means that a vacuum has formed behind your projectile.

When it says "barrel choking flow" it means that the total flow from your valve(s) is higher than the barrel can support - you have possibly got the wrong numbers for your valve in that case.

What is your valve? Someone will know the appropriate figures to sort it out to get you the right results.
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Unread postAuthor: Extrusion » Sat Oct 21, 2006 1:53 pm

my valve is a 1/2" ball valve and i have no ideo how to put it in GGDT and when i think ive got it close it say the projectile only got like 100 fps wich i know is way wrong lol.
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Unread postAuthor: POS » Sat Oct 21, 2006 4:17 pm

barrel choaking flow isn't that bad. I get the same message when I calculate the speed of the projectile and barrel of a smaller diameter than the valve. No woeries about that one. What wories me, is the other message. It indeed creates a vacuum, slowing your bullet down at the end of the barrel, due to lack of pressurized air. You should make the barrel shorter !!
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Unread postAuthor: Extrusion » Sat Oct 21, 2006 5:19 pm

but how much shorter i only have 2 choices a 30" barrel and a 54" barrel i just shot 2 ball berrings into the same tree at the same psi but different barrel and they both went 1 1/2" or so into the tree wich really amazed me lol 1 1/2" into a hard tree thats harsh lol but what i need is someone to GGDT my gun for me because i have no idea how to for the person that does they get 50 spud bucks :D

Specs:
First Barrel 54" long 1/2" wide
Second barrel 30" long 1/2" wide
Chamber 28" long 2" wide
Valve 1/2" ball valve
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Unread postAuthor: POS » Sun Oct 22, 2006 4:03 am

50 ? Nice, But a bit hard, since you only have 10 bux :lol:
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Unread postAuthor: MrCrowley » Sun Oct 22, 2006 4:49 am

1:go to the ggdt website it has info on all the errors
2:why can't you cut the 54" barrel to a shorter length?
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Unread postAuthor: Extrusion » Sun Oct 22, 2006 1:08 pm

yeah i decided to just go ahead and buy the signature ability and i dont have anything to cut a iron barrel with lol
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Unread postAuthor: boilingleadbath » Sun Oct 22, 2006 8:08 pm

I'm running a generic valve, 100 ms opening time, .5" porting, at 70% flow:

Running out of air is not an issue, even running at 3 PSI.
Flow choking is not an issue
muzzel velocity is ~ 300 fps with a 7 gram projectile @ 100 psi from a 54" barrel.
The 30" barrel gives you 225 fps with the same conditions.
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Unread postAuthor: Extrusion » Mon Oct 23, 2006 5:02 pm

Thanks blb is the reason im getting such low fps because of the valve? im pretty sure thats it lol.
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Unread postAuthor: boilingleadbath » Tue Oct 24, 2006 1:55 pm

Yes, the valve is really limetting preformance; barrel pressure never exceeded ~20 psi, IIRC.
I'd expect an enormous improvement if you used a 1" sprinkler. (which acctualy has a bit more flow [~25% more] than your barrel can use, but a 3/4" would limmet preformance... so I'd pay the extra couple dollars and get the more effective product.
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Unread postAuthor: Extrusion » Tue Oct 24, 2006 2:00 pm

boilingleadbath wrote:Yes, the valve is really limetting preformance; barrel pressure never exceeded ~20 psi, IIRC.
I'd expect an enormous improvement if you used a 1" sprinkler. (which acctualy has a bit more flow [~25% more] than your barrel can use, but a 3/4" would limmet preformance... so I'd pay the extra couple dollars and get the more effective product.


Yes i did ggdt on a modded 1" spinkler valve on my gun and i got around 554 fps!! as opposed to the 300 with the ball valve talk about a HUGE increase in performance!
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