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marble autocannon

Built a BB Machine Gun? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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marble autocannon

Unread postAuthor: jackssmirkingrevenge » Wed Dec 30, 2009 10:01 am

Herewith the latest prototype in my series of pop-off auto launchers.

First, some target videos:

[youtube]http://www.youtube.com/watch?v=EYfNtMtwo0Y[/youtube]

The smileys look a bit more worried these days :)


[youtube]http://www.youtube.com/watch?v=x0xVhZhew2M[/youtube]

Another cardboard smiley gets perforated


[youtube]http://www.youtube.com/watch?v=j0lcBjICnVg[/youtube]

A piece of 1/4" plywood, as you can see power isn't exactly amazing.

The heart of the system is a modified 1 inch pop-off valve coupled to a simple tee/detent magazine. I think the low power can be attributed to the fact that the pop-off operates at a maximum of 3 bar (circa 45 psi) and for a relatively short interval, but still, nice to see it operate quite reliably :)

Here's there complete setup:
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Unread postAuthor: JDP12 » Wed Dec 30, 2009 10:17 am

Very nice, I think it's coming along good. Have you given any thought to possibly gutting and modifying the popoff to pop at higher pressure?
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Unread postAuthor: ramses » Wed Dec 30, 2009 10:18 am

Nice. Not too powerful, but I still wouldn't want to be shot by it.

I have to say, though , that a EV+popoff seems much more powerful, as well as compact. Are there any advantages of this, beyond having 1 less part.
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 30, 2009 11:01 am

JDP12 wrote:Very nice, I think it's coming along good. Have you given any thought to possibly gutting and modifying the popoff to pop at higher pressure?


I'm not even using it at full pressure, the problem is that for it to pop, there is a minimum level of flow (otherwise it will just hiss) and since this beast has 1" porting, the flow requirement is too high for my compressor to supply.

ramses wrote:I have to say, though , that a EV+popoff seems much more powerful, as well as compact. Are there any advantages of this, beyond having 1 less part.


If you're talking aboutthis one, psycix is running that at 30 bar, that's 10 times the maximum rating of my valve and for reasons pointed out above I'm actually running much lower than that, I doubt it's popping at more than 20 psi (though I'm feeding 110 psi into the chamber).

The advantage of this system is that it's more efficient as all the air goes through the barrel, it needs to be tuned in terms of spring strength and chamber flow but this is also true of a QEV/popoff system. It really doesn't get much simpler as a constant flow auto valve, unless you use the projectiles as a "valve".
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Unread postAuthor: POLAND_SPUD » Wed Dec 30, 2009 11:24 am

well it is impressive anyway

I don't quite get it... couldn't you simply replace the spring with a stronger one ??
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 30, 2009 11:41 am

POLAND_SPUD wrote:I don't quite get it... couldn't you simply replace the spring with a stronger one?


It's already a very tough spring, at its maximum 3 bar rating it's having to hold back around 15 kilos of pressure. The problem as I explained is that it needs a certain amount of flow to work as an auto valve. It's to do with the amount of air going into the chamber versus the amount of air being dumped by the valve. For this reason I can't currently use the valve at its maximum rating.

One possible solution would be to reduce the chamber size to almost nothing.

Also, I agree wholeheartedly with your assesment of marbles as very poor ammunition, I wish there was a better large calibre ammunition that was in plentiful and cheap supply.

Worth noting too that for a relatively low rate of fire, the tee/detent breech was more than adequate to provide a reliable supply with no double feeds.
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Unread postAuthor: psycix » Fri Jan 01, 2010 2:17 pm

3 bar? Psssh....
Nice work though. The low rate of fire seems to help feeding.

Can't you pressurize the piston (or diaphragm) from behind? (instead of a stronger spring)


3 bar rattlesnake wrote:If you're talking about this one, psycix is running that at 30 bar, that's 10 times the maximum rating of my valve and for reasons pointed out above I'm actually running much lower than that, I doubt it's popping at more than 20 psi (though I'm feeding 110 psi into the chamber).
Currently, the popoff pops at 19 bar. :)
However, I may get some scuba tanks soon and manufacture my own popoff. I'm thinking.... 35 bar popping pressure...
I might also try to use this design instead of pilotting a QEV with the popoff.
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 01, 2010 2:56 pm

psycix wrote:Can't you pressurize the piston (or diaphragm) from behind? (instead of a stronger spring)


That might actually work, I can make a gas piston out of a 10mL syringe that fits instead of the coil spring which will have a 1:3 area ratio to the main piston seat so pressurised to 300 psi it should hold 100 psi... hmmm...
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Unread postAuthor: psycix » Fri Jan 01, 2010 4:03 pm

Knowing you I expect it to be curing the next time you post. ;)
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Unread postAuthor: jackssmirkingrevenge » Fri Jan 01, 2010 4:10 pm

Am I that predictable? Results some time tomorrow evening ;)
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Unread postAuthor: POLAND_SPUD » Fri Jan 01, 2010 4:59 pm

@psycix
scuba tanks soon
have considered using LPG or CNG tanks??

LPG tanks are rated to about 25 bar IIRC (burst pressure ~ 70 bar) good but I would rather have something rated a little bit higher for a fridge compressor... though I have to admit they are really dirt cheap and have large capacity (40 - 120 L)

CNG tanks are rated to about 200 bar !! they also have large capacity but they are kind of expensive (at least in Poland)


I've seen a lot of cheap used scuba tanks but either they are to heavy to be shipped or their capacity is low... are you buying them off ebay ?

@JSR
hmmm why don't you buy an LPG tank and attach it to your compressor ?
that would solve your problems

also I know I suggest it from time to time but you should really build a fridge compressor
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Unread postAuthor: ramses » Fri Jan 01, 2010 10:22 pm

jackssmirkingrevenge wrote:
If you're talking aboutthis one, psycix is running that at 30 bar, that's 10 times the maximum rating of my valve and for reasons pointed out above I'm actually running much lower than that, I doubt it's popping at more than 20 psi (though I'm feeding 110 psi into the chamber).


I have to point out that psycix was feeding it 30 bar, but IIRC, it was popping at around 100 psi (?). It'll be interesting to see how this performs with an air spring.
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 02, 2010 1:05 am

POLAND_SPUD wrote:hmmm why don't you buy an LPG tank and attach it to your compressor?


What I've done is made the chamber smaller (dramatically so, practically tapped the blowgun directly into the valve) so it gives me some leeway when playing with flow rates. We'll see how it performs at 100 psi.

also I know I suggest it from time to time but you should really build a fridge compressor


I'm always on the look out for dumped fridges but no luck yet, in the meantime my 450/600/850 psi fill station is catering to my HPA needs ;)

ramses wrote:I have to point out that psycix was feeding it 30 bar, but IIRC, it was popping at around 100 psi (?)


Actually, from the man himself:

psycix wrote:Currently, the popoff pops at 19 bar. :)


That's about 280 psi, at least 10 times higher than what I'm using.

It'll be interesting to see how this performs with an air spring.


Curing as we speak, I hope it's reinforced enough to take the 300 psi it needs to hold back 100 psi at the seat (and the subsequent pressure spike when the piston pops), we'll see.

edit: might take a bit longer, I was pressurising it to remove the piston, which didn't budge till about 100 psi then shot out into the wall and shattered *sigh*
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Unread postAuthor: JDP12 » Tue Mar 30, 2010 11:01 am

I know this is an old topic, but have you given up on this JSR??
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 31, 2010 12:13 am

Technically it can still be made to work, the air spring was completed and it works, but it needs a bigger chamber to cycle and I never bothered making one.
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