Barrel tightness efficiency experiment

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Postby fullmetaljacket » Tue Feb 22, 2005 3:50 am

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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Postby TakerLabs » Tue Feb 22, 2005 6:35 am

Thats A very informative test. I and others (i'm sure)do thank you. Most of us dont have the test equipment and are thankful when those of you that do, post your results.
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Postby fullmetaljacket » Tue Feb 22, 2005 6:56 am

No problem, that's what a forum is for :D
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Postby Mr.Plow » Tue Feb 22, 2005 8:58 am

Just wondering: how straight did you get the copper tubing? Could any bends in it affect the data? I wouldn't think so, but I'd just like your input.
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Postby fullmetaljacket » Tue Feb 22, 2005 9:18 am

I straightened them by hand, and they are no means 100% perfect, but as close as I could manage :) I'm sure a perfectly straight barrel would give slightly better velocities, but not much better.

<img src="http://pictures.greatestjournal.com/userimg/3350112/512443" width="311" height="425">

There seems to be some distortion in the image, note that the grey bit between the tubes is a steel straight edge but it appears wobbly in the photo:?: If you look through the end of a tube, the two ends appear concentric so it's quite reasonable. The lower tube is the one I used for the tests, the masking tape around the end is to seal it with the blowgun.
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Postby kozak6 » Tue Feb 22, 2005 4:26 pm

You should try a piece of 1/4 inch brakeline as well.
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Postby fullmetaljacket » Tue Feb 22, 2005 5:00 pm

Originally posted by kozak6<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">You should try a piece of 1/4 inch brakeline as well.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

I would if I could get my hands on some, do you think I <i>enjoy</i> straightening coiled copper tubing? :| - however it's sold as 1/4 tubing, so presumably it's got the same 4.5 mm / 0.177 inch internal diameter as the 1/4 steel line that you use stateside?
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Postby syco kilaa » Thu Feb 24, 2005 12:13 am

I can get 1/4'x.035"x36" pure aluminum tubing at a place called burkes brother hardware, just wanted to see if you have any to use in your tests, and this stuff is crazy by the way, it takes a bb about 3 or 4 seconds to reach the bottom of a 18" peice of this stuff with you hand over the end, thats a very tight fit, not really tight but almost all of the sides of the bb are touching. almost...
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Postby fullmetaljacket » Thu Feb 24, 2005 12:41 am

All you have to do is grab a verier caliper and measure the inside diameter, if it's 4.5mm/0.177 of an inch, then the data would be very close to the one I got for the copper tubing.
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