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Working Marble Machine Gun

Built a pneumatic 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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Working Marble Machine Gun

Unread postAuthor: skyjive » Mon Jul 13, 2009 8:40 pm

Here is my new copper Marble Machine Gun. It runs off of CO2 and has an operating pressure of 120 psi. It has a rate of fire of several shots per second and is powerful enough to put a marble through four layers of thick cardboard. It has a pressure tight T magazine with spring-loaded ball stop and a mini blow-forward bolt to prevent double feeds.

A video of a firing of the gun. At around 20 sec into the video the magazine accidently rotates down, which pretty much puts the kibosh on the firing, but most of the 12 rounds loaded in were fired first. Max capacity is about 20. The headphones are to protect my ears from the muzzle and pilot blasts, which were pretty loud.
http://www.youtube.com/watch?v=O0WXDEGQFOk

And here's a close-up of the damage:
ImageImage

Some more pics:

The pilot area. The pilot valve is a 120 psi pop-off valve, and there is a constant CO2 flow through a quick connect at the bottom.
Image

The magazine and QEV. The QEV is 3/4" and the magazine is a pressure tight T design.
ImageImage

The ball stop. The top of the clothespin is fixed in place, and a small piece of 1/8" steel rod goes from the bottom through a hole in the copper into the T. This stops a marble from rolling forward. When the marble is fired the rod is pushed down and out of the way against the resistance of the clothespin spring.
ImageImage

Here is a view down the open magazine:
ImageImage

Looking along the barrel. The wooden cylinder at the front is not part of the gun, just some convenient junk to prop up the barrel with.
ImageImage

The blow-forward bolt:
ImageImage
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Unread postAuthor: FORE!!!! » Mon Jul 13, 2009 9:34 pm

nice machine gun ,,it looks like u have soft soledred ur joints,just be carefull not to bump ur gun around or ur joints will crack and then,well hopefully the joint holds :wink:

basically ur pipe is the strongest part on ur gun where as the joints usally are,,ever used silver solder?
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Unread postAuthor: Copperboy » Tue Jul 14, 2009 12:29 am

Nice! :P For how long can you fire with that bottle?

I really don't think you joints are gonna crack no matter what you do... maybe get bent if one stresses them enough.
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Unread postAuthor: POLAND_SPUD » Tue Jul 14, 2009 8:31 am

is powerful enough to put a marble through four layers of thick cardboard
it seems to be more powerful than that... have you tested it on tougher targets ?


does the blowforward bolt block air passage to the magazine upon firing or not ? ifi t doesn't i tmight be a good idea to get rid of that as it must be hampering performance quite a lot

I've noticed that there is a huge deadspace between the mag and the valve... at least 1 ft/30cm
is it really necessary ? well I am not a efficiency nazi but apart from lowering performance that extra 30 - 40 cm of deadspace makes the gun unwieldy

EDIT
why are you shooting it inside ? and with no eye protection ? :shock:
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Unread postAuthor: skyjive » Tue Jul 14, 2009 10:54 am

I am using silver solder, and have used it for about a year now on various guns without any trouble, why would the joints crack?

The distance between the mag and valve is due to the need for a place for the bolt as well as the fact that I ran out of money halfway through this project and had to recycle some groups of fittings from previous guns, which made the construction less efficient than it could have been.

And no, the bolt doesn't block off the magazine, as I couldn't get it to do so without jamming on the return stroke, but I'm still working on it since that mag makes a lot of wasted volume.

Shooting inside due to lack of anywhere to shoot outside, and come to think of it eye protection would be wise, although my soft backstop hasn't resulted in any richochets yet.
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Unread postAuthor: Technician1002 » Tue Jul 14, 2009 11:40 am

skyjive wrote:I am using silver solder, and have used it for about a year now on various guns without any trouble, why would the joints crack?


The old lead based solder has a bad habit of work hardening, becoming fractured and letting go in joints that have mechanical stress.

The old lead based electrical solder was a problem in high power circuits where thermal cycling was common. The expansion and contraction cycles would fatigue the solder and the leads on components would develop a crack in the solder all the way around a lead leading to intermittent loose connections. Re heating these and re flowing the solder fixed many a power supply, amplifier, VCR, etc. It's the first thing inspected for in a set that has an electrical loose connection somewhere. Headphone jacks are prone to this failure in MP3 players.

Silver bearing solder is much better in that regard, but requires higher temperatures that will burn some cheap printed circuit boards. The strength of the joint when used on copper pipe, I'm not sure.

Silver Brazing solder on the other hand is very strong, and the joint is stronger than the pipe. I've seen A/C systems crack elbows and pipe next to joints while the joint held. This solder is most often used in refrigeration systems since no flux is used. It holds the pressure and there is no flux to damage the compressor or filters on the inside.

You can wash flux off the outside of the pipe or flush out a water pipe, but what's inside a sealed refrigerator is in there for life.

A silver brazing solder joint is easy to identify. It doesn't scratch easily. Regular solder scratches easily leaving a shiny scratch. Silver bearing solder is harder to scratch. It's not soft like lead. Silver Brazing solder is harder than the copper pipe. The pipe scratches much easier. Most all refrigerators and air conditioners are assembled with it. It is so hard, it is not sold on a spool, but as rods.

If you look at the soldered joints on the copper, the solder is bright shiny and sliver color. Lead solder is also shiny but has a grey tint. Lead solder oxidizes quicker than silver bearing solder so it loses it's shine faster.

Edit, Grabbed the camera and took a photo of all three types.
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Solders.jpg
Silver bearing solder is in front. Nice and shiny with a silver color. Behind it is electrical lead tin electrical solder. The copper colored rod is 15% silver brazing solder. On copper pipe it looks a lot like electrical solder in color, but it is very hard. Soft lead based solder will write on paper. The other two will not rub off on paper.
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Unread postAuthor: jackssmirkingrevenge » Tue Jul 14, 2009 1:31 pm

Congratulations on bodging together something that works, however now that the concept has been proved there's a lot of refining to be done, I concur with POLAND_SPUD's comments with regards to efficiency.
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Unread postAuthor: POLAND_SPUD » Tue Jul 14, 2009 5:20 pm

well if the bolt doesn't block off the magazine then the performance is already hampered by volume of the magazine... and if it still works then I suppose you can remove the bolt...

removing the bolt should reduce flow restrictions (I suspect they are worse than a bit of deadspace) and let you cut that lenght of pipe between the valve and the T

any bolt that obscures the flow of air will to reduce performance (sometimes quite a lot)... trust me I've been there


if it already doesn't serve any purpose then getting rid of it should increase power and fix other minor problems (as well as make the gun simpler... and simpler = less things that can go wrong)
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Unread postAuthor: skyjive » Tue Jul 14, 2009 6:33 pm

It would be great to be able to eliminate the bolt with its considerable flow restriction and long housing, but that's all thats preventing the entire magazine from shooting out with a single air burst. I tried a paddle interrupter, but it didn't work and got blown off its hinge after a few shots anyway.

As someone who likes to build auto systems, I fully hear you on simpler=better. In this vein, adding a front piece to the bolt to block off the magazine would add complexity (to a fatal degree, I think). It's a balance between the efficiency of the launcher considered from a single shot perspective and the functionality of the highly ghetto mechanism.
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Unread postAuthor: POLAND_SPUD » Tue Jul 14, 2009 8:32 pm

the bolt doesn't block off the magazine
hmm sorry I understood that the bolt doesn't block off the mag :)

so does it block off or not ? :D
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 15, 2009 1:06 am

skyjive wrote:that's all thats preventing the entire magazine from shooting out with a single air burst


For a Tee mag to work well, the magazine inner diameter should be close to the projectile diameter, ideally it should be the same material as the barrel.
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Unread postAuthor: skyjive » Wed Jul 15, 2009 10:04 am

It is. Both magazine and barrel are 1/2" copper, and the ammo is 1/2" marbles that fit quite closely. By "the entire magazine" I meant "all the rounds loaded in the magazine", the problem being that without something to limit the feed to one round per air burst one burst would fire an almost unlimited number of marbles as as soon as the bottom one was pushed out from below the T more would just keep dropping in and being fired as well.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 16, 2009 12:15 am

Sounds like your main valve is staying open for too long, any chance of fiddling with the pop-off pressure?
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Unread postAuthor: POLAND_SPUD » Thu Jul 16, 2009 1:04 am

or air enters the magazine and then escapes after the marble leaves the barrel (and it carries another marble with it)... lol that's my theory
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 16, 2009 1:52 am

That can only happen if the valve is not opening in short enough bursts, a bit of fidling with piloting/resetting rates should do the trick. That's the frustrating thing with autos or any cyclic mechanism really, getting the balance right :roll: :D
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