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Ammo tolerance in barrel?

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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Ammo tolerance in barrel?

Unread postAuthor: Ianbuckwell » Tue May 06, 2014 11:11 am

I was just wondering what tolerance you guys used/still found efficiently effective?

I am particularly interested in guns that use some kind of feed system, I was thinking of using the tube of ammo with a spring to move them along and keep them packed up and then once the ammo had its turn to be loaded into the barrel I would use a small high power magnet, this would be drilled partially way through the 3mm wall thickness and then jb welded into place.

Any ideas thoughts or possible foreseeable problems, thanks.
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Re: Ammo tolerance in barrel?

Unread postAuthor: jackssmirkingrevenge » Tue May 06, 2014 12:49 pm

As tight as you can, it makes an enormous difference to both power and accuracy.

Here's a test I had done a while ago (holy f... in 2005!)

It's fairly obvious that the closer the barrel diameter is to the projectile diameter, the less air is wasted and therefore, more power. I decided to set up a simple experiment to see just how much of a difference it makes.

The basic setup was two 15 inch lengths of barrel tubing, one was aluminium with an I/D of 5mm, while the other was a straighted piece of coiled copper tubing with an I/D of 4.5mm. The barrels were hooked up to a blowgun attached to a compressor and used to fire lead BBs - with a nominal diameter of 4.5mm - at 100 psi through a chronoscope.

Over a 5 shot string, the 5mm I/D barrel averaged 242 feet per second, whilst the 4.5mm I/D barrel averaged 279 feet per second.

With an average projectile weight of 8 grains, this gives a muzzle energy of 1.04 and 1.38 ft/lbs respectively - a significant power increase of 33% over the wider barrel.
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Re: Ammo tolerance in barrel?

Unread postAuthor: Ianbuckwell » Tue May 06, 2014 2:44 pm

Thanks, you are a fountain of knowledge :) seriously though, thanks. I should have chosen my barrel size more carefully as cheap BB ammo is not available in my size, only expensive proper machine bearings, closest I can get in slingshot ammo size is 9mm I think. Had a look through eBay and found some 8.5mm BB, downside is I think they are on the slow boat from China!!! Not that I have a gun to put them in yet, slowly collecting parts I need, kind of excited, may sound silly but you can go out and buy an air rifle, but this will give me the satisfaction that I have at least assembled it myself, also noting air rifles are restricted to a feable 12lb/ft power, if you want more you need a fire arms certificate, and as you may know in UK we can't just go out and buy a gun like you have the option to.
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Re: Ammo tolerance in barrel?

Unread postAuthor: jackssmirkingrevenge » Wed May 07, 2014 5:58 am

Ianbuckwell wrote:I should have chosen my barrel size more carefully as cheap BB ammo is not available in my size, only expensive proper machine bearings


Be warned that slingshot ammunition is not very consistently sized.
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