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Accuracy?

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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Accuracy?

Unread postAuthor: Foxx » Mon May 04, 2009 6:09 pm

Hey guys, this is my first post here, so go easy.
I have a pneumatic gun
SPECS:
Barrel: 1" PVC ~2.5 feet long
Chamber: 1.5" PVC ~2.5 feet long
Valve: Modded Sprinkler
(Sorry, no pic currently)

My question is what kind of accuracy should I be getting? At 40 psi I can barely make shots with consistent accuracy at ~50 feet. Closer (~25 feet), it is very accurate. Is this what i should expect? Is there a way I can increase my accuracy?

Thanks in advance!
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Unread postAuthor: ckman » Mon May 04, 2009 6:14 pm

put a longer barrel on it
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Unread postAuthor: Foxx » Mon May 04, 2009 6:17 pm

how long does it need to be? how much will that increase the accuracy?
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Unread postAuthor: grock » Mon May 04, 2009 6:43 pm

as a general rule, a longer barrel = more accuracy, untill it is so long that you get barrel droopage, which can be solved by sleeving it in another piece of pipe. it also depends on what you are shooting, molded slugs have a much higher accuracy rating then say, rocks with wadding
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Unread postAuthor: inonickname » Mon May 04, 2009 6:44 pm

Longer barrel, more consistent ammunition and higher pressures. With such low pressures it will begin to tumble very fast.

Your C:B ratio could be around 2:1 for noptimum power, so you may need to make the chamber larger as well.
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Unread postAuthor: twizi » Mon May 04, 2009 6:44 pm

pump it up to 115psi, larger recoil
but it would go at higher velocitys so it would go farther and that means better accurcy but also put a 3 1/2 foot barrel on it that would do a lot better
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Unread postAuthor: Foxx » Mon May 04, 2009 6:46 pm

How much pressure is safe to put into it?
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Unread postAuthor: grock » Mon May 04, 2009 7:07 pm

there should be a pressure rating on the side of the pipe. its varies by manufacture
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Unread postAuthor: Foxx » Mon May 04, 2009 7:08 pm

oh ok, thanks all
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Unread postAuthor: Ragnarok » Mon May 04, 2009 7:13 pm

Ugh. People spouting whatever the hell they feel like.
A longer barrel is not conducive to accuracy. In many cases, a longer barrel can actually worsen accuracy, due to it changing the resonant frequency and amplitude of the barrel.

However, in this case, a slight increase in barrel length might not be overly harmful (although I'd need to see your launcher design to be sure) and would aid velocity, as would some increase in chamber pressure - although, without knowing the exact specifications of your chamber pipe, I cannot tell you how much more pressure is safe.

Do bear in mind though, more velocity does not equal more accuracy. It's essential for shooting at long ranges, but it's not what makes something accurate.

For me to really be more helpful, I'll need to know more information - how accurate is it at 25 feet? What's the smallest target you can hit reliably? How inaccurate is it at 50 feet? What's the smallest target you can hit reliably at those ranges?

The important thing here is that you've used the term "accurate", and different people define "accurate" differently. If my air rifle was failing to hit a 5 pence piece (damn near the same size as a dime) at 50 feet, I'd first check the scope was properly zeroed and then I'd be taking it apart and cleaning it, because I'd consider that highly inaccurate for my air rifle. I doubt you're setting the same harsh standards of your spudgun.

Also, what's your choice of projectile?

Basically, we need to know more to tell you if you can expect more accuracy, and what you need to do to improve accuracy if you are falling short of reasonable expectations.
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Unread postAuthor: Foxx » Mon May 04, 2009 7:17 pm

At 25 feet i can hit a soda can reliably
At 50 feet, i can hit a 1'x1' box reliably
Projectile is a 2.5 in. piece of dowel wrapped in tape to fit the barrel
The PVC is called Charlotte Pipe (i think) not positive though. dont remember what the pressure rating is.
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Unread postAuthor: Ragnarok » Mon May 04, 2009 7:33 pm

It sounds like it's possibly got a problem with the projectile tumbling.
Are the projectiles flying smoothly, or do they start to tumble and spin wildly when in flight?

If they're flying erratically, it might be in that case that you'll need to experiment with projectiles.
Drag stabilisation is quite an easy trick, and it'll probably help out. For a quick experiment, you could try is adding a hollow paper tail to the dowel projectiles.

Basically, if you take a reasonably sturdy tube of paper that has the same diameter as your dowel, but a little longer than the dowel (perhaps 3" long) and tape it in place so that about 2"-2.5" of the paper tube extends off the rear of the dowel, then load this so that the paper tail is... well, at the back.

This is a technique known as drag stabilisation, where the centre of gravity is forwards of the centre of drag on the projectile, and this means it will naturally fly nose forwards without tumbling.

Try that out and see if it helps you at all.
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Unread postAuthor: Foxx » Mon May 04, 2009 7:35 pm

Thanks for the advice, I'll try that. I'll also bring the pressure up to ~60. Hopefully that will make a difference.
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Unread postAuthor: twizi » Mon May 04, 2009 8:45 pm

also you could rifle the barrel that would help and boring out the back of your ammo would also help
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Unread postAuthor: Ragnarok » Mon May 04, 2009 8:52 pm

twizi wrote:also you could rifle the barrel that would help and boring out the back of your ammo would also help

Rifling a spudgun barrel is not a particularly easy task to do well.

Boring out the back of the ammo does exactly the same as what I was talking about with the paper tails, by moving the centre of gravity relatively forwards, but it's going to take more effort and time than the tails.
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