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Basic potato rifle

Built a combustion cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Basic potato rifle

Unread postAuthor: cfb_rolley » Sun Apr 29, 2012 2:35 am

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So I got my newest basic combustion rifle in to a working state today. 760mL chamber forms the stock, multiple barrels, peizo ignition housed inside an old nerf gun trigger handle. At the moment it's only running off spray propellants or spray butane. I use a hand pump with a tube attached to vent, but at the moment I'm having trouble getting a decent fuel mix, so it's pretty unreliable.

Currently I just have the short 30mm bore barrel for portability, and the longer 20mm barrel for longer range/ higher muzzle velocity.

Additions to come are a scope rail, red dot sight, cheek pad, a paint job and possibly a bipod and some more barrels.
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Unread postAuthor: Gun Freak » Sun Apr 29, 2012 1:06 pm

Can we see the rifling? :)
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Unread postAuthor: Fnord » Sun Apr 29, 2012 3:00 pm

Getting the fuel and air well-mixed in that thing is going to be a nightmare. Try a length of 2" pipe with a small fan in it instead. You can keep the stock, but it'll just be a stock from then on.
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Unread postAuthor: cfb_rolley » Sun Apr 29, 2012 8:37 pm

I also thought of having a kind of portable fuel mixer. a peice of 2 inch pipe with a fan inside tapered to a flexible hose that could be fed into the chamber with a few holes in it to spread the fuel. That way I could use it for other small or odd shaped cannons in the future and I don't have to modify this one.
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Unread postAuthor: jackssmirkingrevenge » Mon Apr 30, 2012 12:28 am

Gun Freak wrote:Can we see the rifling? :)


Ignore the smart arse brian :)

Given the chamber size, precise butane injection (25mL) can be done with a syringe.

Given that you have a threaded fitting at the barrel that could presumably take a burst disk, why not add a schrader valve and make a low mix hybrid?
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Unread postAuthor: cfb_rolley » Mon Apr 30, 2012 1:23 am

It is possible to add a burst disc, is it safe to go hybrid with PVC though? Weakest part is rated to 12 bar.
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Unread postAuthor: jackssmirkingrevenge » Mon Apr 30, 2012 1:30 am

I'm definitely not suggesting you should go to high mixes, certainly not more than 2x. I personally would be comfortable with the latter provided all the fittings were well put together and the burst disk would definitely burst at 12 bar.
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Unread postAuthor: cfb_rolley » Mon Apr 30, 2012 1:39 am

Hmm. Well in that case I might take it in to consideration haha. There is also 45mm pipe and from memory rated to 18 bar available to me, and I also still have my burst disc air cannon to play around with different burst discs so maybe I'll do a hybrid similar to this. I did play around with fuel mixes on HGDT after I measured the chamber volume and the results at 1.5x were pretty good, though I'd expect a bit of performance loss due to the bends.
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Unread postAuthor: cfb_rolley » Wed May 02, 2012 4:04 am

alright so I've done the idea with the portable fan to blow air around inside and still no luck with reliability. in fact, I'd say it's now actually less reliable. So I ran the numbers on HGDT again and measured the chamber volume more accurately by filling it with water, and including the space in the barrel behind the projectile it's 685cc (was quite a bit off...)

I was thinking of going fuel metering so I ran the numbers on HGDT and have found it'll give me about a 25% performance increase just by going to metered propane and changing the barrel length.

My question, however, is since I can't fit a chamber fan in there to mix it, where should the fuel enter the chamber from so that it'll spread evenly? The spark gap is in the middle of the T where the top and bottom tubes meet up.
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Unread postAuthor: jackssmirkingrevenge » Wed May 02, 2012 4:10 am

I would put a fuelling port between the two elbows at the back and fuel it muzzle down.
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Unread postAuthor: cfb_rolley » Wed May 02, 2012 4:43 am

Thanks JSR, your wisdom is always seriously appreciated!
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Unread postAuthor: mattyzip77 » Wed May 02, 2012 4:56 am

jackssmirkingrevenge wrote:
Gun Freak wrote:Can we see the rifling? :)


Ignore the smart arse brian :)




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Unread postAuthor: Crna Legija » Wed May 02, 2012 6:31 am

Even a small fan like this would be enough.
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Unread postAuthor: cfb_rolley » Wed May 02, 2012 7:15 am

I could probably scrounge one up from somewhere, where would I place it though?between the two elbows blowing upwards?
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Unread postAuthor: jrrdw » Wed May 02, 2012 7:31 am

cfb_rolley wrote:I could probably scrounge one up from somewhere, where would I place it though?between the two elbows blowing upwards?


Radio Shack and close to where the fuel enters the chamber.
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