V.A.L Piston Valved Antenna-Golfball-Nerf Launcher (video)

[Technician1002]

O ring, wrap in tape, layer of felt, anything that will reduce the blow by but not make the pistion stick.

The Wiki has some information on piston valves and how they work. With a good understanding of how they work, then you can understand why they don't work.
http://www.spudfiles.com/spud_wiki/inde ... ston_valve

[Pibster]

I added a rubber gasket to the rear of the piston. Works great now.
Thanks.

[MrCrowley]

Well here's the cannon after a new upgrade/downgrade as i've removed the stock. Paintjob has suffered a bit and I just realised it has almost been two years since I posted the cannon.

[Pibster]

Was there a noticeable loss of power from reducing the chamber? I like the look, much cleaner looking. Easier to service the piston if needed. I'm assuming the barrel will need to be shortened. I am building two more for my friends, I think I will make one with this modified design.

[MrCrowley]

Was there a noticeable loss of power from reducing the chamber?

Not at all, my chamber was far too large before.

assuming the barrel will need to be shortened.

Never, ever should you shorten a barrel unless you have to. More often than not, it is increasing barrel length and not chamber volume which gives you better performance.

Thanks for the comment.

[Technician1002]

There is a point of diminishing returns on barrel length. For any given barrel diameter and supply pressure, there is a length that will provide maximum muzzle velocity. Too long of a barrel will have too much flow resistance and longer barrels will provide poorer performance.

To understand this, simply look at an air hose connected to a compressor. Using a shorter hose will provide higher flow than a longer hose. In both cases the chamber (compressor tank) is oversize for the small diameter barrel (hose) and the barrel is too long to provide maximum muzzle pressure (velocity).

In a competition, in-barrel acceleration was measured to find the length where the projectile no longer accelerated. The barrel was cut slightly longer than this point because some of the barrel flow restriction was removed. The result was a barrel with higher muzzle velocity and the projectile was still accelerating when it reached the muzzle.

You will always want to have a positive pressure in the barrel as the projectile reaches the muzzle. Otherwise the projectile is slowing down before it reaches the muzzle. I have an acceleration curve showing this in one of our test shots. This can be modeled in GGDT.

PM me if you want copies of some of our test prints.

[MrCrowley]

There is a point of diminishing returns on barrel length. For any given barrel diameter and supply pressure, there is a length that will provide maximum muzzle velocity. Too long of a barrel will have too much flow resistance and longer barrels will provide poorer performance

On this forum what is more likely; that someone would reach diminishing returns by extending their chamber volume or barrel length?
I'm sure you'll agree that most people seem to have far too large of a chamber for such a short barrel. With a decent size pneumatic, it seems you can go up to 10' in barrel length and still get performance increases.

You're quite right with your points, my comment above is more of a general statement. Rarely do you see someone with a diminishing return barrel length compared to chamber volume.

[Technician1002]

I'm sure you'll agree that most people seem to have far too large of a chamber for such a short barrel. With a decent size pneumatic, it seems you can go up to 10' in barrel length and still get performance increases.

In testing a 2.5 inch barrel on the 3 gallon shirt launcher, I was surprised to find the diminishing returns was considerably less than 10 feet at 60 PSI. Ideal was closer to 7 feet on a 2 inch port valve.

Burnt lake did extensive barrel length tests. It was aimed at finding the best C/B ratio, but diameter/Length plays into the best length too.

The guy on Crazybuilders built an air cannon with a massive chamber and Long barrel. I suspect his barrel is too long for the valve size. A shorter barrel will most likely work better.

17 gallon tank, 2 inch valve, 10 foot long barrel. My 3 gallon outperforms it. His shots are less than 500FPS Mine are greater than 600FPS.

Photo crasybuilders.com