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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: judgment_arms » Wed Mar 05, 2008 9:51 am

After seeing the results from my Wookie gun I’ve come to the conclusion that pure, un retarded, blow-back will not work well in this application.
And blow-forward designs seem to work, but they do it to fast, and with out a retarding mechanism I see them continuing to do so. One way around that would be LPHV, low-pressure high volume, as the delay well the chamber fills would undoubtedly retard the firing rate, but it could also retard power.

Jack and Punkin, one thing you need to consider is the fact that your bolt only has to move a fraction of an inch, which means it’s most definitely going to have a high rate of fire do to do to the short distance things have to travel and the speed they tend to do it at.

I feel that we’re on the right track; we just took the side road that parallels the interstate of automatic fire. :)

I believe the key was figured out in the late 1800s to early 1900s by John M. Browning in the recoil and gas operation principles.
Recoil wouldn’t work to well in small caliber arms, gas operation, on the other hand, shouldn’t require to much, pressure ways, to make it work, just a large diameter gas cylinder or high pressure operation.
Another thing to consider is one of the many delayed blow-back mechanisms, but I’m not to thrilled with any form of blow-back.

As for the design posted by Jack, i think you’ve got something there, it basically a hammer valve, same thing I’m going to use, but the problem with hammer valves is they produce incredible ROF, I took the grip frame, and trigger group, off a paintball gun once and let it run, it went “Brrrrrruuuuuuuuup” sounded about like 1500+ RPM maybe close to 1800 RPM.
One thing you might consider is some form of gas delay mechanism that unlocks the barrel-bolt-hammer assembly once the projectile passes the gas port. That would increase power also.

That’s my 2 cents.
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Unread postAuthor: Sticky_Tape » Wed Mar 05, 2008 10:44 am

jackssmirkingrevenge wrote: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.


Well maybe you could do the proto as a lil activity on the side couldn't be too hard for you dink .
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 11:50 am

judgment_arms wrote:As for the design posted by Jack, i think you’ve got something there, it basically a hammer valve


The first one is like a hammer valve, but this design is more akin to a pop-off valve, and this is in fact the beauty of the idea - you can delay the rate of fire (ie the amount of time it takes for enough pressure to build up inside the chamber to pop the bolt) by changing the airflow into the chamber, but no matter what this rate is, the valve will always fire at the same pressure, giving you reasonably consistent performance.

One thing you might consider is some form of gas delay mechanism that unlocks the barrel-bolt-hammer assembly once the projectile passes the gas port.


With the pop-off variation there's no need for such a delay mechanism, all you need to do is choke the flow to the chamber.

Well maybe you could do the proto as a lil' activity on the side


I guess I could, not building anything else at the moment anyway.

Gah, I hate you for bringing this up! For some reason after having promised myself to leave these things alone, I now feel compelled to try this out :? :roll:
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Unread postAuthor: Sticky_Tape » Wed Mar 05, 2008 12:11 pm

I guess I could, not building anything else at the moment anyway.

That's the spirit! O yeah and the spring you could use could be from a really cheap airsoft spring pistol.They normally have strong springs.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 12:23 pm

I feel another epic fail *points to icon* coming on but hey, if at first you don't succeed :roll:

I mean so far, this has been my progress in the auto department:

Image

:D

...but enough of this playful banter, I have futile measurements to make and epoxy to mix!
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Unread postAuthor: dewey-1 » Wed Mar 05, 2008 12:46 pm

--
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 1:00 pm

Thanks for the encouragement ;p well I still have these bits left over from a previous attempt so I'm already off to a good start, all of a sudden it seems a shame to have come this far in terms of design refinement and not actually giving this a go.
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Unread postAuthor: Skywalker » Wed Mar 05, 2008 2:26 pm

Attaboy, Jack! :P Way to prototype our crazy ideas for us! Well, actually, this was *your* crazy idea, wasn't it? lol
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 3:01 pm

Skywalker wrote:Well, actually, this was *your* crazy idea, wasn't it? lol


It's a combination of the blow-forward breech, which wasn't my idea, and the pop-off valve, which is definitely not my idea - but I don't think anyone's put them together yet :)

I'm putting it together slightly differently however, something like this:
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Unread postAuthor: Sticky_Tape » Wed Mar 05, 2008 3:08 pm

Are you going to put somthing on the poppet rod to stabilize it in the firing camber? I don't know but maybe the support could have holes resticting some of the flow to slow the rof.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 5:10 pm

That's just a conceptual diagram, I'm not exactly sure how I'll go about it in practice but you'll be sure to have diagrams when it's done.

In the meantime, the bolt casting is curing, but there's a fair amount of castings left before I can see if it works, or doesn't :roll:
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Unread postAuthor: judgment_arms » Wed Mar 05, 2008 5:42 pm

jackssmirkingrevenge wrote:but this design is more akin to a pop-off valve


Jack, I don’t know, it seems to me that it would get stuck (attached picture shows were/when). You see that main spring has to be strong enough to hold the valve open long enough to build pressure, but week enough to let the pressure close the bolt and fire it. I think that when the bolt/hammer comes back and opens the valve to fill the chamber and then the bolt moves forward, once it gets to where the air is dumped it will drop the pressure enough for it to close, as it closes the chamber volume decreases and pressure builds back up to were the spring pressure and air pressure balance out and the valve becomes stuck.
Am I right?

And Jack, I disagree with you, it’s going to have to be a hammer valve in order for the bolt/hammer to nock open the valve, with all the pressure on the valve holding it shut I don’t think you can just push it open, it’ll take a bit of force.

By the time I got all this typed I noticed that you’d posted a new design, one that looks better, but I’m still seeing the same problems.
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Unread postAuthor: Sticky_Tape » Wed Mar 05, 2008 6:07 pm

I have a Idea That I think you should use JSR it's about the poppet valve rod you know the green thing it should be a bit longer maybe 1-4th way down the place where the piston travels this lenth should give more time for the chamber to pressurize but maybe the bolt wouldn't be able to come back far enough for the bb to load so the bolt rod might have to be shortened.I think it would work.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 05, 2008 11:14 pm

spudchucker wrote:I have a idea that I think you should use JSR - it's about the poppet valve rod, you know the green thing - it should be a bit longer maybe 1/4th way down the place where the piston travels, this length should give more time for the chamber to pressurize


Ah, but when it comes back it will open the valve before the piston on the bolt has time to seat and thus will perform exactly like the previous blow-forward designs.

judgment_arms, remember that this is a pop-off valve - the spring only has to resist the pressure acting on the small area of the port, meaning it can be relatively weak. One the piston starts to move and its full area is exposed, the available surface area increases by 500% with the dimensions I have, meaning there's 5 times more force to overcome the spring strength - so the hope is it will gather enough momentum not to "hang" open.

It's going to have to be a hammer valve in order for the bolt/hammer to nock open the valve, with all the pressure on the valve holding it shut I don’t think you can just push it open, it’ll take a bit of force.

Well, when the system is at rest, there is no air to hold the valve closed - you need so supply it from a blowgun or ball valve, so it will already be open. For the second shot, the spring will be pushing the bolt back with reasonable haste, presumably this will have a "hammer" effect on the valve. Speaking of which I was thinking of using a schrader for this purpose...
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Unread postAuthor: judgment_arms » Wed Mar 05, 2008 11:40 pm

Maybe, but I still feel the valve will just stop. Like I said as the bolt moves forward it’ll get to the point that it’ll open a crack and then pressure will drop allowing the bolt to return which decreases chamber volume which in turn increases pressure, it’ll fluctuate for a second (figuratively of course) and then stop.
Remember, your not going to have to much momentum on the forward stroke as it’s proportional to how fast you fill the chamber, which will be slow due to the fill valve.
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