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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: SubsonicSpud » Thu Apr 09, 2009 4:00 am

a copper pipe with a severe case of constipation, get some fiber into it :lol:
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 09, 2009 4:30 am

SubsonicSpud wrote:I have to say I might try making a large high pressure one for a marble sized gun, definitely has a nice rate of fire.


I'm thinking of scaling it up for a marble auto myself to be honest, it will give me something to do until the HPA bottle arrives :D but with a say 3/4" piston seat to pop at 80 psi it's going to need a 16Kg preload on the spring, so construction would have to be substantial to say the least. There are commercial large pop-offs you can buy but I can make one for a material cost less than 10 euros, so not too keen on spending over 100 to buy a ready made one.

I don't want to be bleeding buckets of air like a pop-off repeater.


Precisely why I embraced this design instead of following the QEV+popoff philosophy ;)

Carl(wo)man wrote:now thats a keeper mate! love the 'bulge'


You would, ya big poofter :D

(Wasn't there some Aussie sports car that gave the same audio warning when you engaged traction control? I think I had seen it on Top Gear :D)

a copper pipe with a severe case of constipation, get some fiber into it


I put some steel into it, look what happened :roll: :D
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Unread postAuthor: SubsonicSpud » Thu Apr 09, 2009 4:56 am

jackssmirkingrevenge wrote:
I'm thinking of scaling it up for a marble auto myself to be honest, it will give me something to do until the HPA bottle arrives :D but with a say 3/4" piston seat to pop at 80 psi it's going to need a 16Kg preload on the spring, so construction would have to be substantial to say the least. There are commercial large pop-offs you can buy but I can make one for a material cost less than 10 euros, so not too keen on spending over 100 to buy a ready made one.


So a a 18mm seat @ 400psi means..............I'm going to have to find a nice big spring :twisted: all 130kg's of it. I might make a air spring to get that sort of spring rate :lol:
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 09, 2009 5:00 am

An air spring would definitely be an easier way of going about it, but it will require maintenance to keep it airtight. Also, if you're using high pressure, the outlet doesn't have to be the same diameter as the barrel for good performance. I think you could get away with a 12mm seat and 10mm outlet.
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Unread postAuthor: SPG » Thu Apr 09, 2009 5:05 am

I'm wondering with the HPA tank, do you actually need the chamber? Would it work just taking the air directly from the tank/regulator directly to the piston face?

Just a thought.
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 09, 2009 5:22 am

SPG wrote:I'm wondering with the HPA tank, do you actually need the chamber? Would it work just taking the air directly from the tank/regulator directly to the piston face?


My thoughts exactly. I was thinking tank -> ball valve (trigger) -> pop-off unit - barrel

I'd have a Tee magazine in a projectile diameter tube, containing 50 or so BBs and spring loaded, that should be a lot more compact and with a lot less dead space than the hopper, as well as being more reliable. A sprung detent would also be a better idea than the o-ring I used in the hopper design. The tank would double as a pistol grip to have a "realistically" positioned magazine ahead of the trigger. Alternatively I could have it in line with the barrel, either as a butt stock logun s-16 style or with the base facing the muzzle to give a P90 style bullpup, I think the latter option would be more appealing. Technically the magazine can be in line with the barrel too, giving a very slick outline.

I think 450 psi should suffice for my purposes, I apparently will have the option to go to 850 with the inbuilt tank reg but I don't think the increased power would justify the increased stress on the system, noise and air usage.
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Unread postAuthor: SPG » Thu Apr 09, 2009 5:34 am

Jack, just a thought on some form of bolt. In this design your bolt's open while the projectile's heading off down the barrel and so you still get air going into the magazine.

Image

But how about if you used something like this camera cable release.

Image

connected behind the piston so that as the piston opens the magazine is shut off? I imagine a small barrel sized cylinder acting as a bolt, which would seal the magazine, but with a pin push the BB over the port without blocking it.

Think Brian's T-bolt repeater but automated by the piston action.
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Unread postAuthor: SubsonicSpud » Thu Apr 09, 2009 5:59 am

jackssmirkingrevenge wrote:An air spring would definitely be an easier way of going about it, but it will require maintenance to keep it airtight. Also, if you're using high pressure, the outlet doesn't have to be the same diameter as the barrel for good performance. I think you could get away with a 12mm seat and 10mm outlet.


Yep I was thinking along the same lines. I would probably not fabricate an air spring from scratch, been there done that, and they are a pain in the butt to seal properly. Would probably mod a existing cylinder of some description to get a reliable seal. Actually a clutch slave cylinder would make a good, relatively cheap air spring 8) the ideas are slowly coming to me. Time to raid the Landy spare parts me thinks
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Unread postAuthor: SPG » Thu Apr 09, 2009 6:33 am

I've just been eyeing up a spare gas strut I've got from a car hatchback. I wonder how much force is needed to get that compressing.
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Unread postAuthor: psycix » Thu Apr 09, 2009 6:44 am

jackssmirkingrevenge wrote:
Is that a bottle on a T-piece? And it doesnt double feed like hell?
Yes it is, and no it doesn't, because of the way it's designed:
<image>
With each blast of air, the BB in the breech goes out of the barrel while the rest are pushed up the magazine tube.

Okay, since reading that I thought: Drop that darn blowforwardfuck. If its so simple, then why am I spending so much effort into making a damn BFWB

So I also did it with a simple T...
I can tell you, results were pleasing... :roll:
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Unread postAuthor: SubsonicSpud » Thu Apr 09, 2009 6:51 am

SPG wrote:I've just been eyeing up a spare gas strut I've got from a car hatchback. I wonder how much force is needed to get that compressing.


probably not much more than about 30-50kgs for a car hatchback but they come in a huge range of pressures. The problem with the gas lifters is they have a built in pneumatic dampening. You have to bleed the gas and oil out very carefully and seal up the metering orifice in the piston. A schrader valve could then be installed into the bottom of the cylinder to pressurize to the required amount
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 09, 2009 9:04 am

SPG wrote:But how about if you used something like this camera cable release connected behind the piston so that as the piston opens the magazine is shut off? I imagine a small barrel sized cylinder acting as a bolt, which would seal the magazine, but with a pin push the BB over the port without blocking it.


I see what you mean. Heck, why not make it a hydraulic system, with a wide piston (for which read syringe ;) ) at the valve and a thin piston at the bolt? That way, small movement on the valve end is amplified to give you maximum bolt travel. Eg a 2cm diameter piston at the valve and a 1cm piston on the bolt will give you 4mm of bolt travel for every 1mm of piston travel.

SubsonicSpud wrote:Time to raid the Landy spare parts me thinks


You have a Landy too? :D

psycix wrote:So I also did it with a simple T...
I can tell you, results were pleasing...


Then show us! yeeesh! You're the biggest tease I know, and you don't even have breasts!

:roll: :D
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Unread postAuthor: POLAND_SPUD » Thu Apr 09, 2009 9:15 am

So I also did it with a simple T...
I can tell you, results were pleasing...
I've already figured out that bolts = problems

the funny thing is that I discussed with JSR about a loading mech based on a T long long time before I started working on jackhammer/air-ram designs... lol I should have tested that then

my semi works ok but the power is hampered becasue the blow-through bolt casues too much obstruction... I know I could build a better one or build the vertibolt but right now I am considering switching too lower caliber (BBs) and implementing david's hopup unit (I think you should consider doing the same)

ohhh BTW my fridge compressor is somewhat broken - I don't know what the problem is but it takes 1h for it to start up... I have some ideas how to use compressors without starting circuits but that would take some time

lol sorry I know it's not ITWOST thread... maybe there should be one dedicated to semiauto/full auto guns... InTheWorldOfSemiauto/FullautoSpudguns
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Unread postAuthor: SPG » Thu Apr 09, 2009 9:22 am

Ok quick thought on home made air-springing for this. I know the problem with them is that the leak if you don't watch out, but all this talk of clutch master-cylinders got me thinking. What about an air-oil system?

If you filled the area behind the piston with oil and then have a remote reservoir higher than this with air, voila, you've still got an air spring, but the oil should hopefully leak far less round the piston than air. A bit of experiment with different viscosities should give something manageable, but throw a ball valve in the connecting tubing and you can adjust the flow and hence the rate of fire. Then there's all sorts of playing with check valves that's possible to give you different flow rates on the outward and return strokes.

Possibilties for fine tuning seem endless.
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Unread postAuthor: jackssmirkingrevenge » Thu Apr 09, 2009 9:27 am

I would say don't worry about leaks, have it 99.5% airtight and just top it up to a given pressure before every firing session.
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