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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: psycix » Tue Oct 20, 2009 4:08 pm

Jack, adjustable IS better. You can be glad that it popped open so you get to fix the silicon seal.
I'm currently playing around with something that loads bb's just the way I want... adjustable of course.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 20, 2009 4:48 pm

Looking good there :)

Actually, I've been thinking on a tangential line... since I can retain a BB in this tubing at such high pressure, and seem to have gotten good at blowing stuff up, perhaps that hybrid cartridge...

Keep an eye on the obituaries ;)
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Unread postAuthor: Lentamentalisk » Tue Oct 20, 2009 10:06 pm

why not just make the ID of the silicon seal slightly larger, so that the BBs will slip through at a lower pressure? It seems like the silicon seal would be better than a ball detent, because the BBs will actually form a seal against it until they pop through, rather than letting the air slip past.

Good idea on the hybrid...
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 21, 2009 12:32 am

Lentamentalisk wrote:why not just make the ID of the silicon seal slightly larger, so that the BBs will slip through at a lower pressure? It seems like the silicon seal would be better than a ball detent, because the BBs will actually form a seal against it until they pop through, rather than letting the air slip past.


Also an option, I have located a suitable material, however adjustability is definitely a desirable feature as psychix pointed out. I imagine the best solution is combining both detent and seal. You can also make the seal adjustable, jimmy had suggested an idea here whereby the seal is externally pressurised, determining its grip on the BB.

Image

No idea how that would hold out at 850 psi though, not many things I make seem to these days :oops:

On another vaguely related wavelength (around 240-254nm in our industry...) does anyone know what sort of pressures a 20x hybrid mix with butane would generate?
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Unread postAuthor: Lentamentalisk » Wed Oct 21, 2009 1:09 am

well the rough estimate of 100psi per "x" suggests on the range of 2000psi.
At that pressure, I am sure you could get even an entire elephant to fit through your little silicon seal...
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 21, 2009 1:43 am

Lentamentalisk wrote:I am sure you could get even an entire elephant to fit through your little silicon seal...


That almost sounds like flirting :wink:

Hmm, 2K psi eh, maybe we should start with a 5x mix and work our way up... I can make an experimental cartridge to keep my hands busy while my next BBMG is curing :D
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Unread postAuthor: Lentamentalisk » Wed Oct 21, 2009 3:14 am

I was going to go for the standard "thats what he said", not so much flirting...

Anyways, I don't know how accurate the 100psi per atmosphere estimate is. It is based on the maximum combustion pressure of air/propane around STP. How that changes as you get to crazy pressures, and change to MAPP, I don't know, but I would suspect it would still be in a similar range. Also, as you get to higher pressures, the heat transfer to the walls of the chamber will increase, so that will also lower the effect.

Interesting note:
While propane hybrids are limited to around 20x, if you "hybridize" them further, by using both propane and MAPP or butane fuels, then you can get to much higher mixes, as it is the partial pressure of the specific chemical that maters, not the total pressure, when dealing with liquification.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 21, 2009 5:02 am

Image

I'd be pressurising with my shock pump for accuracy so about 20x would be the limit. Ok so here's the plan, make one standalone cartridge, I'm thinking 0.177" and with a 3/8" brass body, and see how that goes. If nothing else it will be the world's smallest hybrid, another achievement to add to my list of tiny-launchers-that-emphasise-how-I-really-have-nothing-to-compensate-for :D I'll do a couple of low mixes, if that words out before moving on to higher mixes I'll male a proper breech and barrel setup.
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Unread postAuthor: psycix » Wed Oct 21, 2009 5:17 am

While propane hybrids are limited to around 20x,

Wasn't that 200x? Might be mistaken, not sure about it so feel free to correct.
I believe butane would limit somewhere between 20x and 50x, which you can add up to the propane mixture if using both.

JSR, be VERY careful with that hybrid thing... you survived an 850 psi catastrophe, but that doesn't mean you will survive 2000...
How could we live without our epoxy-prototyper...


Nice idea to use it for the hybrid cartridge you've been wanting to build for so long.
Don't you think the seal will just break when the bb goes through?
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 21, 2009 7:14 am

psycix wrote:JSR, be VERY careful with that hybrid thing... you survived an 850 psi catastrophe, but that doesn't mean you will survive 2000...
How could we live without our epoxy-prototyper...


Happily with standard fittings ;)

Don't worry, this will be an incremental build-up process, and besides we're talking about a very small scale effort here (cartridge around 8x50mm) though of course it only takes one well placed little fragment to cause disaster.

Nice idea to use it for the hybrid cartridge you've been wanting to build for so long.
Don't you think the seal will just break when the bb goes through?


Aye, if I can make a shell-ejecting prototype with the same power as my high pressure pneumatic minis it will be quite a milestone. Using a bit of metal rod I can poke a BB through the seal with relatively little force, and it's quite resiliant stuff we use in the lab so it should be ok. As always, testing will tell, I figure I can make it this evening and have it ready for trials by tomorrow.
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Unread postAuthor: psycix » Thu Oct 22, 2009 5:59 am

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

Then why didn't the BB go through at 850psi?
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Unread postAuthor: jackssmirkingrevenge » Thu Oct 22, 2009 7:04 am

No idea, tiny area for the air to push on?
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Unread postAuthor: CpTn_lAw » Thu Oct 22, 2009 8:57 am

I think you could need to put the seal - in some kind of way - further in the barrel. What I think happens is that the air rushes fast around the BB, creating an air cushion between the seal and the BB.
You might say "stupid...think about cloud BBMG's..." Yes, I did. there is a difference (slight but still...) in barrel diameter where the BB goes in.
In a cloud, it's most likely the barrel that is poked into the chamber; in your case, the diameter is slightly smaller, therefore a small cushion of high pressure air (or CO2 whatever..) is created to compensate for the venturi effet.
Imagine that with your case, the high pressure gas you're using pushes the BB towards the seal, and as it goes, the space between the seal and the BB dicreases, air accelerates around the BB creating a depression around it, having less "force" (this term isn't used right. ) to push it in the barrel's direction. I'm not making myself clear...god I need a little time off ....
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Unread postAuthor: Lentamentalisk » Thu Oct 22, 2009 11:44 am

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.

But I don't actually know what was happening.
@JSR: when you tried it and it didn't fire, was it just getting clogged, so no air would escape, or did the BBs just rattle around, letting all the air go? If the latter is true, you may need to bevel the silicon seal, to help feeding.
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Unread postAuthor: Ragnarok » Thu Oct 22, 2009 12:42 pm

Lentamentalisk wrote:While propane hybrids are limited to around 20x, if you "hybridize" them further, by using both propane and MAPP or butane fuels, then you can get to much higher mixes, as it is the partial pressure of the specific chemical that maters, not the total pressure, when dealing with liquification.

The answer for propane alone is about 220-225x. It does depend on temperature though.

Anyway, I first discuss the concept of propane/butane mixes here:
Ragnarok wrote:A mix with 2.93% propane and 0.97% butane added (2.82% and 0.94% respectively by total volume) could be compressed to 320x or so without any of the fuels becoming liquid.

I was guestimating on vapour pressures at the time though, so I'd need to go back and figure it more precisely.

Still, if I were going for a really powerful hybrid, I think the He/H<sub>2</sub>/O<sub>2</sub> mix I proposed a while back (for hypersonic velocities) is probably a good thing to look at.
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Last edited by Ragnarok on Thu Oct 22, 2009 12:43 pm, edited 1 time in total.
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