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Auto-Ram BBMG

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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Auto-Ram BBMG

Unread postAuthor: kablooie » Wed Aug 20, 2008 2:34 am

This is a bbmg I made inspired by the talk of using a homemade air engine to fire (and run) a gatling gun. I decided to scale it back a bit, and instead of rotating barrels I used a very simple (and easy to make) blow-forward bolt design. Here are some specs:

Air supply: 10 lb CO2 tank, or an air tank and and a bike pump
Barrel: 1/4" brakeline
Ammo: .177 cal bb's
Bolt: 1/4" compression hose insert, inside of a 1/4" compression tee
Ram: 1/4" bolt with some tape wrapped around it
Valve housing: 1/8" brass nipple

Basically, this gun works like a mini engine, and uses the exhaust to fire it. You can also think of it as a pop-off valve that vents air into the barrel. The blow-forward bolt was extremely easy to make, I got the idea from RC naval warships that sometimes use this method in their guns. This gun took a little over half a day to make, including buying parts.

The gun definitely needs some fine tuning, like a spring or wieght in the magazine, higher pressure, and different rubber band adjustment, but so far the power has been mediocre. I think it could be greatly improved though. I could start with the dead space I have...

Here's a video, please excuse the quality, this is my first video I've ever uploaded.
[youtube]http://www.youtube.com/watch?v=kpXF0RGNNBk[/youtube]

Here are some pics and if you need any more info, just ask.
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Attachments
Picture 3.png
The Piston/ram thing, notice the rubber bumper ring and bolts for adjustment
Picture 2.jpg
The ram housing, ram and bolt housing, please excuse the tape, this is only a prototype (but could be refined).
Picture 1.png
The full gun in all it's glory...
Picture 4.png
Here's a diagram
Picture 4.png (24.89 KiB) Viewed 2547 times
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Unread postAuthor: POLAND_SPUD » Wed Aug 20, 2008 3:17 am

interesting design... why have you built it in this way ? to lower the ROF?

it seems to that it could work better if you use a strong spring instead of that rubber band and/or get a heavier ram
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Last edited by POLAND_SPUD on Wed Aug 20, 2008 5:35 am, edited 1 time in total.
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Unread postAuthor: trollhameran » Wed Aug 20, 2008 4:39 am

Seems to works very well, except for the feeding, is it just gravity fed?
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Unread postAuthor: ALIHISGREAT » Wed Aug 20, 2008 5:10 am

from the diagram it appeared that the ram goes into the mag.... but it doesn't :shock:
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Unread postAuthor: POLAND_SPUD » Wed Aug 20, 2008 5:34 am

@alihisgreat yeah I've noticed that too... interesting...

@kablooie are you planing to have a bolt operated by the movement of the rod ?? that's the first thing that came to my mind when I saw it
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Unread postAuthor: littlebro05 » Wed Aug 20, 2008 6:14 am

POLAND_SPUD wrote:interesting design... why have you built it in this way ? to lower the ROF?

it seems to that it could work better if you use a strong spring instead of that rubber band and/or get a heavier ram


Wouldn't a light ram be better in a piston since the lighter the piston the faster it recipricates? and the faster the velocity the ram is moving at transfering the energy into the bb?

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Unread postAuthor: trollhameran » Wed Aug 20, 2008 6:30 am

But also the faster it is reciprocating, the higher the rof will be, which makes loading the bbs harder.
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Unread postAuthor: littlebro05 » Wed Aug 20, 2008 6:50 am

trollhameran wrote:But also the faster it is reciprocating, the higher the rof will be, which makes loading the bbs harder.


Ummu, you're right. Thought about the rate of fire didn't think about the loading process.

Oh well I don't know much about pistons anyway. Just a thought.

Edit: Did you're gun break or pierce through that paper, I was just curious.
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Unread postAuthor: POLAND_SPUD » Wed Aug 20, 2008 8:33 am

yeah that what I meant....

I am suprised that it works so well - JSR have been trying to build something like this to get pulsed air but AFAIK with no success... I am sure he is going to be interested in the results..

his piston/ram failed becasue it stayed in an open position - it didn't reciprocate (is it the right word ?? :D ) I see that your does the same occasionally but I suspect that it works better than his becasue it has greater mass....

if it's not a problem could you experiment with it by adding more mass to the ram/piston - just to see what will be different
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Unread postAuthor: trollhameran » Wed Aug 20, 2008 8:41 am

When I made a ram for a gatling prototype, it stayed forward as well. it was almost impossible to push the ram back in at 115psi.
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 20, 2008 8:48 am

POLAND_SPUD wrote:I am suprised that it works so well - JSR have been trying to build something like this to get pulsed air but AFAIK with no success... I am sure he is going to be interested in the results.


If you're referring to this project, it actually did reciprocate, but too fast to be useful in terms of power or practicality. Clide reported similar results with the blow-forward bolt in his golf ball semi while he was experimenting with it. After endless hours of using different spring strengths and lengths as well as bolt weights, I still got the same results - it either reciprocates too fast - the same "angry wasp" effect - or not at all.

This leads me to conclude that a reciprocating system will not work well for our purposes unless the moving component also shuts off the air supply. This element has also proved to be necessary in compressed gas motors like this CO<sub>2</sub> motor, where the pin on the piston head knocks open a check valve providing a "pulsed" supply.
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Unread postAuthor: POLAND_SPUD » Wed Aug 20, 2008 8:48 am

hmmm I suppose that it shouldn't be any different at 115 or 300 psi but there has to be more force reseting it and possibly more mass

was it with the same setup or did you use a stronger rubber band or a spring ? I suppose you didn't becasue I don't really understand why I won't work at higher pressures...

Is there any reason (apart from what I mentioned) why it can't work at higher pressure ??


@JSR hmmm I see..most likely you are right but I had an idea when I saw the GIF with that engine...
wouldn't a flywell work for this ? in theory it should preserve some energy that can be then used to move the bolt back (with the help of a spring)

well his design works - of course not perfectly and with little power but it does.... and It's good to see that there are people who spend time trying to build something new....
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Last edited by POLAND_SPUD on Wed Aug 20, 2008 8:58 am, edited 1 time in total.
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Unread postAuthor: psycix » Wed Aug 20, 2008 8:54 am

Nicely done!
Its a great achievement that you got it to work (even if just a little).
The ROF should be lowered.
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Unread postAuthor: trollhameran » Wed Aug 20, 2008 11:14 am

Would it slow down at all if you increased the bolts friction, maybe have rubber sides on your bolt so that it doesn't slide as easily, or perhaps greatly increase the length of travel, so that is takes longer to move back and forward.
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Unread postAuthor: POLAND_SPUD » Wed Aug 20, 2008 11:29 am

hmmm it's very unlikely that it will help... the problem is that the friction will also slow it down while it resets, which is something you want to avoid...

now I suppose that it could work better if it had a flywell - at least in theory.... but that's just my thought

now what jsr has posted is based on a proven design so it should work

but hey that piston is asking to be used... it would be cool if it rotated something no mater what :wink:

ohh BTW kablooie... kudos for trying to do something special
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