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 Post Posted: Tue Jul 15, 2003 11:00 pm 
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Originaly posted by [Gus],
I have always been intrested in spud guns so i built a combustion chamber spudgun, now a year later I started reading about the air powerd ones and i decided to make one. I made it and went to test fire, loaded the spud, and it only shot like 30 feet. I have a bike pump that tells the pressure and it was at 90 psi. I think the problem is my air chamber length which is 2" thick by 2 feet long. Please help spudgun guru's. Thanks.


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 Post subject: we need more info.
 Post Posted: Wed Jul 16, 2003 12:00 am 
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Originaly posted by Bartman007,
what size is the barrel? how long is it? what kind of valve are you using? etc... if possible pictures would be nice

Peace,
-Bartman007


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 Post subject: the rest of the info
 Post Posted: Wed Jul 16, 2003 9:00 am 
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Originaly posted by [Gus],
The barrel is 2 1/2 feet long, and i use a 3/4inch ball valve


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 Post subject: The chamber is fine, but
 Post Posted: Wed Jul 16, 2003 9:00 am 
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Originaly posted by andeh,
the valve is definately bottlenecking it. If you can't open it very fast too, I can guarantee that's your problem. Unless you have a small bore barrel... but I'm assuming you shot a spud.


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 Post subject: poor performance...
 Post Posted: Wed Jul 16, 2003 10:00 am 
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Originaly posted by IzzyDoesIt,
A 2" x 24" chamber with a 30" long barrel (1", 1.5" or even 2") with a 3/4" ball valve should give you great performance shooting spuds.
1. Check pressure with a tire gauge, it takes an awful lot of pumps with a tire pump to get up to 90 PSI. I suspect your pressure is low.
2. The ball valve has to be opened quickly for best performance. Also, I would go with a 1-inch, or better yet a modified sprinkler valve.
3. You did not mention it, but any leaks, obviously would have to be fixed.
I.D.I.


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 Post Posted: Thu Jul 17, 2003 9:00 am 
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Originaly posted by pacogoatboy,
with a pneumatic gun, you want the volume of the chamber to be at least the same as the volume of the barrel, or better yet, 2 or 3 times larger. within limits, longer chamber/barrel combinations keep the tater in the gun longer, allowing it to be accelerated by the air pressure for a longer time. if the diameter of your barrel is the same as the chamber, then reducing the barrel diameter, even by 1/2", will make a huge difference. :-) the volume of a 2" section of pipe is 2.5 times as much as an equal length of 1.5" pipe.

when the valve opens, the pressure begins to equalize within the gun, so larger the ratio of chamber to barrel volume, the better performance you will have. this is limited by a number of factors, most importantly the size of the aperture that the air is moving through. the potato is out of the gun in a tenth of a second or less if everything is working properly, so if your valve doesn't open completely almost instantaneously, its effective crosssection is reduced dramatically and the gun's performance will suffer accordingly. slow speed can effectively turn a 3/4" ball valve into a 1/2 or smaller valve.

though a ball valve that small should turn quickly, you really might want to look into a sprinkler valve. a 1" valve is between $15-25 at a home supply store like lowe's or home depot, and it takes out all concerns about valve speed for anyone who isn't interested in building a howitzer. they can be modified to fire pneumatically, but for starters, three 9 volt batteries wired to the leads does the trick nicely. they are pressure rated to 80 pounds, unregulated, or 150 pounds, regulated (usually, obviously you will need to check whichever one you choose.) good luck.


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