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9mm SMG project + other HPA BBMG experimentation

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: CpTn_lAw » Thu Oct 22, 2009 12:42 pm

Lentamentalisk wrote:actually, it is the other way around. That is the very principle on which the cloud works... As the air accelerates around the BB, the pressure drops, due to the venturi effect. That will help the BB get lodged in the silicon.


I know the principle behind the workings of a cloud, i made one myself, and :D , as I said, I didn't make myself clear...
We had a simulation like that in physics at the uni last year. Reducing the diameter increases speed of the air at this very point, but when something blocks the flow to a certain point (BB approaching near seal) turbulences can be observed, preventing anything of this exact diameter to completely block the flow. The reason why it works when an O-ring is stuck in a groove inside the barrel is that all the pressure pushes the BB from behind, as where here, I think when the BB approaches the seal, turbulences create a cushion of gas counter-acting the forces behind it (think opposite application point) enough at least to prevent the proper functionning of the device.
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 22, 2009 12:52 pm

What was happening is that a very slight amount of air would exit the barrel until a BB entered the seal, then the chamber would seal up completely, and not depressurise until I opened the reloading screw (or the chamber blew up :roll: ) and I would find a BB jammed in the seal. Anyway, here's the current prototype, all stainless 2mm wall parts made in a smaller tee so there would be less total pressure on the epoxy plugs and with a looser seal (4mm I/D, the one I was using before was 3.5mm I/D). I am - with great strength of will, and because of a natural reluctance to be stuck with shrapnel - leaving it another 24 hours to cure as the weather's a bit colder now and the reaction doesn't happen as fast. Internally apart from the additional mag tube it looks like this.

Keep watching this space :)

edit: gave it a quick whirl at 100 psi to address my concern that I had gone to the other extreme and made the seal too loose not allowing enough pressure buildup, but no worries there, BB got sucked in an it sealed up perfectly :D
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Unread postAuthor: psycix » Thu Oct 22, 2009 4:13 pm

jackssmirkingrevenge wrote:No idea, tiny area for the air to push on?

I believe it pushes a 9mm BB with ~37 kg of force at 850 psi.

I can poke a BB through the seal with relatively little force

Less then 37 kg?
I think you should measure that.
You can figure out your popping pressure.



Well the above prototype seems to look good!
I'm watching this space!
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 22, 2009 4:26 pm

psycix wrote:I believe it pushes a 9mm BB with ~37 kg of force at 850 psi.


*cough*we'vemovedonto4.5mmnowsothatsaround25%ofthat*cough*

A 2 litre bottle of water - around 2Kg - placed on a rod in the barrel couldn't push a BB out of the exploded prototypes, so it was pretty tight. Worth pointing out though that so far I haven't had a breech hold 850 psi for long enough to see if the BB was pushed out, and the last one I made where the top blew out had no BB stuck in the seal so it must have been shot out.

We'll know tomorrow if it works as good as it looks :)
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Unread postAuthor: Lentamentalisk » Fri Oct 23, 2009 12:00 am

Out of curiosity, JSR, did you roughen up the insides of the Tee before making the epoxy plugs? Polished steel is pretty damn hard to get a grip on.

Also, though it doesn't seem to be your problem, seeing as how the plugs pop out, not shatter, you could get some serious structural support by layering in some fabric into the epoxy.

All stuff you probably know, but also for others who try to follow in your footsteps
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 23, 2009 12:20 am

Lentamentalisk wrote:Out of curiosity, JSR, did you roughen up the insides of the Tee before making the epoxy plugs? Polished steel is pretty damn hard to get a grip on.


Naturally, my favourite method in this case is to use the tip of a broke file in my drill, works a treat with hard materials.

Also, though it doesn't seem to be your problem, seeing as how the plugs pop out, not shatter, you could get some serious structural support by layering in some fabric into the epoxy.


I've had 2 fails at 850 psi so far, one was due to the epoxy being unreinforced and unsupported which caused it to shatter like a grenade. The second one was due to my not allowing sufficient time for full curing, indeed of the three plugs in the tee it was the one I had poured last which popped out. Mea culpa, mea culpa, mea culpa, I cannot fault the materials if it is used properly. In the case of the above prototype which I intend to test this afternoon, it will have had 48 hours of curing time so it should be fine.

All stuff you probably know, but also for others who try to follow in your footsteps


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Unread postAuthor: psycix » Fri Oct 23, 2009 1:21 pm

4.5 mm... but actually, it is less, since the seal is 4mm.
You should really measure the force needed to push it through, it would be very useful to see what pressure you will actually need.

Also, when surrounded by epoxy the seal might have problems expanding, thus not being able to let the BB through, or making the required force even higher. But I'm sure you already kept that in mind.
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Unread postAuthor: Brian the brain » Fri Oct 23, 2009 4:40 pm

I'm quite sure pushing a bb through at 1 atm is not the same as pushing one through while the seal is being pushed against the bb at 850 psi.

The grip on the bb increases with pressure jack..
Your epoxy had no choice but to fail...

Image

Well now..there's your problem!!
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 23, 2009 8:16 pm

Good points guys, here's how testing went:

The breech happily did not explode, but it was only after some 15 seconds on the trigger that a BB actually squeezed out, so *idea* - I'll pour a bunch of BBs into the barrel and push them through to loosen the seal. What I didn't allow for was the fact that I made the seal in such a way that it would resist pressure towards the muzzle very well, but not the other way round, so basically I forced it out of its housing, leaving the barrel unrestricted.

When I went to try it out, the effect of a relatively high flow at 850 psi was basically similar to lobbing a small bucket of BBs at the target, BRAAAAAAAAAPPPP!!! :roll: :D most of the little frickers ricochetted back into my legs, fun.

In conclusion, screw seals, adjustable detent it is. The tee appears to be sound so I'll see if I can convert it by drilling out the existing barrel. At least I know feeding isn't a problem hehe. It will be a good distraction while hybrid cartridge prototype version 2 cures :)

edit: detent made, epoxy poured, 100 psi results some time tomorrow afternoon :)
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Note the detent screw is set at an angle so it can emerge from the front, avoiding having to drill through the tee for a perpendiculat setup like on my previous prototypes
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Unread postAuthor: psycix » Sat Oct 24, 2009 1:24 pm

Note that a good detent will also give it a spin. If done right, this means increased accuracy and range!
Does the screw push a hard object in front of the BB with a spring (like I have seen in many diagrams) or does it rely on friction with a piece of rubber (my favorite)?


With a good fit, who cares about a little blowby when it's held back for a split second? No seal needed.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 24, 2009 1:32 pm

Basically and angled version of the original idea, quick sketch of what it looks like internally:
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Unread postAuthor: toogers » Sat Oct 24, 2009 2:17 pm

where would i get one of those pressure washer handles on their own? but, great work! how did you depress the pin valve inside the co2 tank? 8)
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Unread postAuthor: dewey-1 » Sat Oct 24, 2009 2:55 pm

toogers wrote:where would i get one of those pressure washer handles on their own? but, great work! how did you depress the pin valve inside the co2 tank? 8)


Here is some USA sources;

http://www.pressure-washer-parts.com/sh ... igger+Guns


http://www.northerntool.com/webapp/wcs/ ... 2+37+97633
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Unread postAuthor: toogers » Sat Oct 24, 2009 7:38 pm

thanks!









too short.
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Unread postAuthor: toogers » Sat Oct 24, 2009 7:53 pm

wow, that site helped more than i could ever imagine. thanks, i would die without that website.
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