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My dad tells me that to gain power ( pneumaticly) if ur chamber is say 3" u have to then have something like a 2" barrel otherwise it will have no power behind what so ever ? is this true ?

And if it is true would a convertion from 3" down to 2.5" be enough

Are those legnths or diameters you are talking about?

diameters thanks

well it is only half inch difference but then again i got a 2" to 1.5" cannon to work but the camber was as long as the barrel so you may have to use more pipe then thought to get the best outta the cannon!

It really doesnt matter how big your pipes are diameter wise. I have a cannon with a 2 inch chamber and a 1/2 inch barrel. It restricts flow alot but it still works. No offense, but i dont think your dad knows what hes talking about.

In pneumatics you just want to try and make your chamber and barrel volume the same. There are things around here describing the formula to figure that out.

so ur saying it would work if i just had an extremely long chamber

There's no direct correlation between the chamber and barrel diameters. The valve device you use to trigger has much more to do with actual projectile performance as it is the ultimate limiting factor on real air flow between the two.

You'll find several examples of pneumatic guns where the chamber and barrel are the same diameter...even a few where the chamber diameter is smaller than the barrel.

It's really a volume thing...c:b ratio chamber volume/barrel volume. c:b ratios from 1:1 on up past 4:1 are successfully used on pneumatics. The higher ratios however, tend to just waste a lot of air that's blown out after the projectile has well left the muzzle. The heavier the projectile the more air you want in the chamber. For potatos and golfballs, 1:1 - 1.5:1 is more than enough.

Also the higher the chamber pressure the higher the performance, it's about that simple assuming c:b ratios are reasonable. GGDT is a good tool to model your design. Maybe you can give it a whirl.

lol no offence taken my dad has no idea what he's talking about

It depends on the length and pressure you are going to be going to. Its all about figuring out what the volume of the chamber to the volume of the barrel. Thats the C:B ratio. It can change depending on the pressure.

What you are giving us isn't quite enough. You have to tell us the lengths of the barrel and chamber.

Edit: wow, a lot of people posted at the same time.....

noobyspud wrote:lol no offence taken my dad has no idea what he's talking about

He's probably thinking about hydraulic pressure multiplication. Not in play here.

well if ive got ..... 3" diamiter chamber which is 3.2ft long ( 1 meter ) then a reducer down to 2.5" then a 180 bend which leads to a 1ft of 2.5" pvc pipe then a ball valve ( 2.5") then a 4ft barrel. and pressure would be 100psi

would that work

noobyspud wrote:well if ive got ..... 3" diamiter chamber which is 3.2ft long ( 1 meter ) then a reducer down to 2.5" then a 180 bend which leads to a 1ft of 2.5" pvc pipe then a ball valve ( 2.5") then a 4ft barrel. and pressure would be 100psi

would that work

A 2.5" ball valve is a whopper and expensive...you may want to consider a sprinkler or piston design.

You can figure the volumes on your own. V = radius * radius * pi * length....but yes these dimensions would be reasonable. Be sure to calculate the space in the your bends.

This would make for a fairly bulky and heavy gun for holding, ok though if you'll shoot it mounted. I would recommend somewhat of a scale back maybe 2/3 to 3/4 this size. Easier to hold. A tennis ball is going to shoot only so far anyway...too much drag on them.

kk thanks. but im affraid sprinkler valve in the uk is a no no due to the fact that one one i need costs 100 pounds which is nearly 200 bucks.

ive got a 2.5" ball valve for about 40 bucks

noobyspud wrote:kk thanks. but im affraid sprinkler valve in the uk is a no no due to the fact that one one i need costs 100 pounds which is nearly 200 bucks.

ive got a 2.5" ball valve for about 40 bucks

The sprinkler wouldn't have to be the full 2.5" model. You could use a common 1" sprinkler valve in there and get respectable, [not ultimate] results...especially for a tennis ball shooter. I'm sure those are much more affordable, even in the UK.

Also, I don't recall ever seeing a 2.5" ball valve. Largest I've seen here in Spudfiles is 2"...and even it was a whopper. If you make it with the 2.5" you may set a record....

What size sprinkler valve did you have in mind?

I have a modified 1.5" one for sale.

Edit: It's the one in the last post on this thread..

http://www.spudfiles.com/forums/short-l ... 13923.html