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Another Full Auto BBMG idea and prototype

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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Unread postAuthor: punkin » Mon Dec 17, 2007 8:42 pm

Ok...
I wasted, err....INVESTED...most of my day today screwing around with the prototype. I was VERY frustrated with the results. I was even more frustrated to come inside tonight to read that JSR had virtually identical results with a different design, same concept.
Before I divulge more into my results, let me say that my engineering background consists of flunking out of engineering school about 12 years ago...calculus just didn't make any sense, and wasn't any fun to me. However, I have prided myself on "real-world" engineering that I usually am pretty good at.
That being said...this crap doesn't make any sense!

I felt strongly that one of the problems with JSR's designs is that he continued to try and use a blowgun as his air source. Blowguns are typically terribly restricive, so I decided to forgo that. Instead, I placed a 1/4" quick coupler where the diagram showed "gas in" to ensure that I had maximum airflow. Of course, this meant not having any "valve" or "trigger", but the idea is to cycle under constant airflow, so I wasn't too worried about that at this point.

What I discovered...depending on spring pressure, either the piston would not move at all, or it would blast forward and not come back.

What I thought was full-auto cycling yesterday was actually a "bump-fire" result as I started to place the air source on the coupler. In fact, I was able to achieve semi-repeatable results by gently holding the receiver in one hand, just barely retaining it with my left index finger, and gently pushing the air hose against the coupler. I got a result similar to what I believe was Clide's post from way back of his GB cannon cycling ridiculously fast.

Using the "bump fire" method, I cycled a number of rounds, sometimes cylical (1 bb per fire) sometimes multi-fire (multiple bbs per fire). I would guess velocity was pretty good, possible as high as 500fps at times, as I was unable to physically see the bb leave the barrel. However, it was not the result I was looking for.

For you engineering / physicists / NASA scientist / ultra brain surgeon types out there........... WTF?

1.) It would seem that there should be some sort of equillibrium that could be reached between air pressure, piston size, and spring weight that would create the cycling that I desire. However, I was unable to achieve what I wanted from 10psi to 150psi. This makes no sense to me. If the piston won't move at 10psi, and it blasts forward and sticks at 100psi, then logic would state that at somewhere between 10psi and 100psi, the piston would cycle continuously...why doesn't it?

2.) Notwithstanding the above, is it logical that I should be trying a HIGH pressure, low volume approach? (High PSI, low CFM?) should I be trying LOW PSI, High CFM?

Finally...I believe that we are trying to recreate a real firearm, and as I have discussed with JSR in numerous emails, I think he and I are both proving that lightweight projectiles do not have the mass necessary to generate the required breech pressures to properly cycle the kind of action we are playing with.

I realize I'm a newbie to posting in these forums, but based on my research and real-world tests, I think it would require a more mechanical action such as Caselman's, coupled with extremely high PSI air source to make an idea like this practical.

I'd love to hear your thoughts!

--Scott
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Unread postAuthor: Eddbot » Tue Dec 18, 2007 3:18 am

that's a surprising result, you would think there would be an equilibrium pressure in between there somewhere, i guess there isn't, or there is and we can't figure out what's going wrong :fart: don't ask, i'm bored
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Unread postAuthor: Antonio » Tue Dec 18, 2007 3:20 am

punkin wrote: I think it would require a more mechanical action such as Caselman's, coupled with extremely high PSI air source to make an idea like this practical.


You know that the Caselman design is nothing like an autmag(blow forward) design. Its rather a stacker tube system. You should check my latest post on that. I made an airgun with the stacked tube design but I but my operating pressure is too low for automatic fire.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 18, 2007 3:35 am

I'm sorry to hear we got similar results :?

I still believe this can work, after all both you, clide and I got it to cycle so the potential for full auto is there. The crux of the matter is slowing down the cyclic rate to allow for practical fire.

From my experiments, I think the only way to achieve this would be to dramatically increase the weight and travel of the bolt, in order to slow down its acceleration to managable levels as well as providing it with the momentul necessary to complete its cycle.

Personally, I'm inclined to take an alternative route - keep the breech mechanism as it is, and put the time and effort into making an air supply that provides pulses of air. This will entail a bit more complex construction (but nothing I can't handle - though the increasing curing time of epoxy in this cold will be frustrating :cry: ) but at the end of the day will probably give better performance as well as a more reliable mechanism. I hope to start on it tonight, watch this space.

by the way, it would be nice to see some pics of your prototype :)

lightweight projectiles do not have the mass necessary to generate the required breech pressures to properly cycle the kind of action we are playing with.


A valid observation in the case of blow-back actions, but the blow-forward mechanisms we're playing with function independently of projectile weight.
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Unread postAuthor: punkin » Tue Dec 18, 2007 10:29 am

A valid observation in the case of blow-back actions, but the blow-forward mechanisms we're playing with function independently of projectile weight.


But...? Wouldn't higher chamber/barrel pressure also assist the spring in forcing the piston back to the "rest" position? That was my thought, maybe I'm all wet on this one? :?

--Scott
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 18, 2007 10:52 am

A valid observation, but I doubt that it would make much of difference in practical terms because it would only speed up the cyclic rate as far as i can tell.
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Unread postAuthor: ammosmoke » Tue Dec 25, 2007 3:47 am

Maybe this has already been considered, but what about a weaker spring? Am I missing something? Or am I the first to think of it?
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 25, 2007 10:06 am

A weaker spring tends to get blown forward and held forward by the airflow.
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Unread postAuthor: tansit234 » Tue Dec 25, 2007 10:10 pm

Its possible to make a one shot air block to empty a burst into the blow forward bolt. I haven't built it, just a schematic somewhere but it could be tuned.
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Unread postAuthor: VH_man » Tue Dec 25, 2007 10:25 pm

wasnt there someting somewhere that some guy made that shot airgun BB's fully automatic with a constant airflow?

i have NO CLUE where it was, but it ran off of a fire extinguisher, and was constructed out of a hunk of clear plycarbonate. it used an empty 12g Cartridge as the magazine.

i remember the dude saying his worked flawlessly.........
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 26, 2007 1:33 am

VH_man wrote:i have NO CLUE where it was, but it ran off of a fire extinguisher, and was constructed out of a hunk of clear plycarbonate. it used an empty 12g Cartridge as the magazine


You mean this one?

The problem with that design personally was that it was too wasteful of air, and used magnets, limiting the ammunition to steel BBs. Essentially it's a blowback design similar to this, that uses a bolt to alternately block and open the air flow. What makes it work well is the fact that he uses a larger diameter piston for his bolt that allows a greater force to be placed on it by the air and therefore cycle reliably.

I had come up with a similar modification for mine but never built it:

Image

Its possible to make a one shot air block to empty a burst into the blow forward bolt. I haven't built it, just a schematic somewhere but it could be tuned.


Not sure if I understood correctly but the point of this development to to provide a "pulsed" air supply to be used in conjunction with the blow forward breech.
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Unread postAuthor: Sticky_Tape » Wed Mar 05, 2008 8:38 am

Sorry to bring this up but maybe it would work if you you hade a smaller diameter piston same spring and a valve at the back of the cylinder I think it's a poppet JSR had a concept of one and a rod on the piston to trigger the poppet. The smaller diameter piston is for the cylinder to build up more pressurebecause of less surface area for the air to act on. And if you keep the same spring the piston wouldn't be as prone to come back over the piece of pipe that puts air into the barrel and seal it off prematurely.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 8:56 am

I had come to the conclusion that there are two ways in which this design could have been made to work, though I never tried them out for myself as actual prototypes:

Image


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Unread postAuthor: Sticky_Tape » Wed Mar 05, 2008 9:01 am

I think you should make a proto of that JSR I think it would work.
If I were you and I had the materials I would do it for sure and I would do it with good feeling.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 9:49 am

Maybe some time in the distant future, it would certainly be within my material capacity to build - however I'm focusing on perfecting the cartridge idea at the moment.
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