SpudFiles

Hi there, just looking to get assistance with a homebrew design.

I want to make a semi-auto bb gun. I want to run it off 12g co2 cartridges, and keep it as compact as possible.

Attached picture is my design. the parts as labeled by the colored squares;

green - a 12g co2 cartridge
blue - a homebrew adapter that screws onto the co2 cartridge. it will basically be a fitting with a hollow tube in the center that will pierce the cartridge.
yellow - an 'explosive air valve' as per this link: http://www.instructables.com/id/Airgun- ... lve/#step1
brown - a solenoid to push back the nut on the valve. Pushing this back allows air to flow through the center tube of the valve. A barrel will be slotted over this center tube on the exhaust side.
red - standard gravity fed T - magazine. If there is enough room around the solenoid, I would like to make a detachable magazine that serves as both a detent and a magazine, with the detent being a spring loaded bb pressing against an o-ring, and the magazine feed tube being behind that, so the bb's come up behind the detent. If I do this for aesthetics I'd like the magazine to come up from the bottom. just means spring loading the feed tube as well.

my main issue is, with the co2 pierced, will it dump the whole cartridge into the small expansion space behind the valve, or will it expand some of the gas up to 800 psi (the point where co2 becomes liquid) and as small amounts are let out through the valve, let out more gas to replace it? slowly emptying and allowing multiple shots? The expansion space will be all brass fittings with teflon tape and epoxy so should hopefully be co2 proof provided it doesnt go crazy.

I also have a small co2 tank with a simple on-off turn valve on the side. Will this work in the same instance? allowing me to detach the tank once i've closed the valve on the side? obvioulsy a different adapter would be used but I wouldnt have to worry about piercing it.

Sorry for the rampant typos. I'm pretty tired and got too lazy about halfway through this post to correct them.

Oh also I'm using the solenoid because I want the trigger to be remotely located for aesthetics.

you might want to use a balanced hammer valve (or at least partially balanced)
look it up

POLAND_SPUD wrote:you might want to use a balanced hammer valve (or at least partially balanced)
look it up

Poland, he was asking if his design was feasible, as opposed to asking for improvements/further research on it.

The design looks ok(ish), it would work but it would be extremely inconsistent. The co2 being unregulated, and with the valve being quite inconsistent due to the level of precision involved (i'm presuming you don't have a machining shop at your disposal) the capsule would be using more/less gas per shot than necessary, especially with no system to fine tune or adjust it.
Also feed reliability is probably going to be quite shoddy, as currently you have no system to assist feeding such as a blow forward bolt.
I'm afraid your going to have to rephrase your questions though, as they are it's kind of difficult to make out what your asking.

the solenoid would dump all the air unless you have a very quick pulse circuit that you will also need to produce at least 20lbs of force from the solenoid, this sort of task usually uses a hammer valve of some sort where only the inertia of the hammer opens the valve so it only stays open for a brief moment and the mass of the hammer and spring tension has to be carefully calibrated, the solenoid would force you to use a minimum amount of mass

you also would want to keep the cartridge oriented verticaly due to the small expansion volume

cammyd32 wrote:Poland, he was asking if his design was feasible, as opposed to asking for improvements/further research on it.

just looking to get assistance with a homebrew design

I think POLAND's reply can be considered 'assitance'.

MrCrowley wrote:

cammyd32 wrote:Poland, he was asking if his design was feasible, as opposed to asking for improvements/further research on it.

just looking to get assistance with a homebrew design

I think POLAND's reply can be considered 'assitance'.

I was more Interpreting it as 'assistance in assessing the projects feasibility' given the discussion title and opening line, but I see your point.

haha thanks for the help guys. I can see what you mean with the inconsistent shots. I was hoping to keep the opening time and distance uniform by using the solenoid with the exact same voltage applied each shot etc, but I see with the extra force required that might be hard to regulate. I stumbled across that valve while researching hammer valves, you are very correct in assuming I dont have access to a machine shop, and so the extreme simplicity of that valve appealed to me. I will keep researching hammer valves and try and find one im capable of assembling with my limited tools and most frustratingly, parts.

So, in continuation of the train of thought of my previous post, I've been working on trying to build one of the simpler designs here. It's JSR's 'valveless' design as used here: http://www.spudfiles.com/forums/6mm-mag ... 21684.html

I've managed the valveless part alright actually, by using an o-ring squished between two fittings. More squish = more shot pressure. At the moment I'm only capable of supplying 60psi through an aquarium regulator I acquired a while ago. My problem is actually in the chamber. I made a chamber out of galvanised steel, essentially;

blowgun > 1/4" nipple > 1/4" - 3/8" adapter > 10mm - 15mm reducer > 15mm steel tube > 15mm endcap. every part is steel, except for the alu blowgun, which has a steel valve and a wall thickness of about 3mm.

in the 15mm endcap, I drilled out a hole and tapped it, and then threaded my schrader valve. I used teflon tape on every fitting and also epoxied where the schrader valve goes into the endcap. My aim was to use a co2 inflator to fill the chamber with an unknown pressure (safety yay! I know its at least sub-800 PSI. The blowgun leaks at 500psi and that didnt happen so I assume this wasn't passed). When I did this the schrader made a loud pop and leaked. I swapped out the valve core and got the same result, although since the leak was slow I still got a wicked shot off with it. no ammo at the time so didn't get to test the chambers capacity.

Is there a max psi to be on the 'inside' end of a schrader? I didn't notice anything out of the ordinary with the valve core I removed, and I assumed higher pressure would just make a schrader more effective as it pushes it closed harder. I think the 'pop' was just my hand getting pushed off the valve by the pressure, I wasn't holding the inflator on with much force.

Is there a max psi to be on the 'inside' end of a schrader?

Yeah, it's about 10,000 psi
No joke. I recall reading that a senior member tested one to that pressure once. It sounds like what is happening to you is that your core just isn't screwed on tight enough. What are you using to screw it in? Pliers won't cut it. I made a custom wrench out of a broken drill bit and a scrap of aluminum that I drilled 2 holes in. If only I could find it...

hah, wow. must be the case, I was using pliers. Do they come from the factory properly installed? I scavenge mine from bicycle tubes if that makes a difference. If they're good from there I'll just buy another and not fuck with it this time

I've never bought a Schrader valve in any condition but used. Sorry, I can't say whether factory bought schraders will be better or not. Just make a key like the one I wrote about, it takes 5 minutes.

It was Gippeto who hydro tested a shrader, it fused, but never leaked. And the shrader tools are about $2 at a bike store, I've even been given a few.