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Refinements wanted

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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Refinements wanted

Unread postAuthor: Penhorse » Wed Sep 05, 2012 6:41 am

So it started as a casual bet between me and my brother as to who could build the best one. Here's mine.


http://www.youtube.com/watch?v=8CI32h6f ... ata_player

It's pneumatic, this is at about 60 psi.

The valve is rather clever though I say so myself. Looks like a standard ball valve eh? Not so! There is, twixt ball and barrel, the capacity to put a burst disk. One layer of foil holds about 20 psi (very ish) so a triple layer means that when the ball opens, the gun doesn't actually fire until enough air has passed through the ball to burst the disk. Which, I hope, removes the early trickle loss of air.

There is an earlier hilarious video of this system failing terrifyingly but I'm afraid I can't post it because the cameraman dropped the f bomb after soiling himself. Hence the cowering in this clip.

I thought I was looking good! Then brother Paul, who lives on a farm, sent me this

http://www.youtube.com/watch?v=yMmCndKo ... ata_player

It's a piece of field artillery.

Now I'm pretty sure I can take him on accuracy. But range and hang time? I'm stuffed here arn't I. Bringing a gun to a cannon fight!

I've tripled the size of my chamber and with a better footpump I might make 70 or 80 psi, is there anything else I can do?
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Unread postAuthor: Crna Legija » Wed Sep 05, 2012 8:57 am

use steel and go hybrid!
to actually improve your cannon get a way longer barrel, chamber barrel ratio of 0.8-1 is what you should look for.
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 05, 2012 10:16 am

I note that you're probably in the UK, so it's best to exercise caution in the public eye as legally a barrelled device capable of more than 12 ft/lbs of muzzle energy is considered a firearm, illegal posession of which carries a hefty penalty.

Burst disc or not, the ball valve is the limiting factor.

You can consider a two stage burst disk: http://www.spudfiles.com/spud_wiki/inde ... burst_disk

Image

A simpler idea would be something like this:

Image

You would need an air source like a compressor (not a big investment really) to make the above practical though. Since you're just mucking around, the small delay between valve opening and disk bursting should not be significant :)
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Unread postAuthor: Penhorse » Wed Sep 05, 2012 10:35 am

How does one measure muzzle energy? I didn't even think of the legal aspect! Has anyone ever been arrested for making a spudgun?!
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Unread postAuthor: Penhorse » Wed Sep 05, 2012 10:37 am

Surely fig 2 is basically what i've already made, only the blow gun trigger is replaced by the ball valve!
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Unread postAuthor: jackssmirkingrevenge » Wed Sep 05, 2012 11:47 am

Penhorse wrote:How does one measure muzzle energy?


http://en.wikipedia.org/wiki/Muzzle_energy

I didn't even think of the legal aspect! Has anyone ever been arrested for making a spudgun?!


Not to my knowledge, but if an annoying neighbour reports some nutter shooting stuff in his back garden, you can expect a visit from the plod. The point is, be careful and don't stick you neck out.

Surely fig 2 is basically what i've already made, only the blow gun trigger is replaced by the ball valve!


If I got it correctly, your burst disk is between the ball valve and the barrel. This means that the ball valve is still a restriction. What I am proposing is barrel -> burst disk -> chamber -> ball valve -> air source, which allows for much better flow independently of how quickly you open the ball valve.
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Unread postAuthor: mattyzip77 » Wed Sep 05, 2012 12:52 pm

Very clever. May I suggest not holding the spud gun against or close to your Johnson if you dont want to hurt it...... :D
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Unread postAuthor: Penhorse » Wed Sep 05, 2012 1:26 pm

I was the man standing behind the other man! My Johnson is going nowhere near that thing!!
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Unread postAuthor: jimmy101 » Thu Sep 06, 2012 3:13 pm

Penhorse wrote:How does one measure muzzle energy?
The easiest way is to measure the muzzle velocity with a round of known mass. The muzzle energy is (1/2)(mass)(velocity)^2. If mass is in grams and velocity is in meters/second then the result is in Joules.

Check the spud wiki for easy ways to get the muzzle velocity.
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