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Wyz recently attempted something along the lines of that , and even with machine tools, and quite a well thought out design it had limited, if any, success.
In theory it should work, but in reality at pressures under around 300 psi, gas escapes too fast to have any useful effect on the hammer.

Given that you are looking for something that can be built without the aid of machine tools, I'm afraid you are going to have to sacrifice small size, and will have to make use of a great deal of off the shelf parts.
Have a look at this design .
Made out of simple pvc without the aid of machine tools and seems to be quite reliable.
Ideally the design is hooked up to a regulated paintball tank, I don't know what you were planning on using as a gas source, but all factors considered the easiest way of achieving portability for a fully automatic system without the aid of machine tools is to use a large compressed gas source such as a HPA or co2 tank.

As I have found, on board compressed air supplies work, such as ones built into the body of the gun, but they take up quite a considerable amount of room on the gun, and also take an eternity to fill up and only last good for a hundred shots max, not ideal for an airsoft game, takes reloading to a whole new level.

Also it's worth noting that, nearly every non-machined gas system, are very heavy on gas consumption, as most are based on; QEV, pop off, vortex and cloud systems, all of which 'waste' some gas, through porting and the like, hence why a large gas reservoir is needed.

Ouch. Reality strikes again. Ok, given that you say that the gas escapes too fast, how does the paintball gun manage it?

If you look very carefully at the diagram the valve stem is stepped, when the valve hits the hammer gas can flow forward of the valve towards the barrel and towards the hammer.
Because the operating pressure of paintball guns is around 300psi the hammer does move a smidgem, (also due to the force on the valve stem from the spring pushing the valve shut) at this point the slightly larger portion of the valve stem pushes into the small hole that allows the gas to flow rearward towards the hammer, (this is the reason for the strange look of the valve)
This separates the two halves of the gun, the paintball is fired using all the gas that is in the power tube at the time, while simultaneously the 'blowback gas' as it is called, cannot escape through the barrel and therefore forces the hammer back accelerating it enough to cycle fully.

All in all it's quite a clever design. Also you have to appreciate that these systems run at pressures in excess of 300 psi, which even if the gas is being released quite fast, is still exerting quite a large force on the hammer, even if for a small time.


The 'valve stem', showing the small step that separates the firing gas and the blowback gas when fully closed.

So basically you are saying I need two expansion chambers. One to expand out the barrel and the other to push back the hammer. They are filled at the same time, then separated as the hammer pops back due to the spring on the valve. Yes?

Methinks I need a more detailed schematic. Suggestions?

Suggestion.... Buy a AEG or a GBB. You will save yourself so much aggravation and time. This project won't be cheap anyhow, and there are several propane-powered gas-blowback airsoft guns on the market now that don't cost a lot of money. The slight difference in price is worth the hassle...

You aren't going to build anything as reliable as a store bought gun... Especially without machine tools.

No offence, Gun Freak, but you aren't quite correct there. A good AEG (In my terms) would cost upwards of 200$. I have already built decent airsoft spring guns without breaking the bank.
Frankly, I'll learn more from building it myself, I'll be able to fix it, and I'll be able to tune it.

Also, I can't justify a 200+ dollar expense. Air is cheap. If I have to, I'll just go with a QEV semi auto, but I was researching my options. I can build a top grade QEV semi auto for around 50$, with a built-in hand pump for topping off the load. As for reliability, AEG's below the 100$ mark aren't that tough; our games are in pastures and woods seperated by fences, high speed play with fast tactics. The airsoft guns have to be very, very tough.

mako wrote:No offence, Gun Freak, but you aren't quite correct there. A good AEG (In my terms) would cost upwards of 200$.

Not true. There are great AEG's that cost under 200 dollars.
For example:
This one for 140 http://www.airsoftgi.com/product_info.p ... ts_id=2163
Or this one for 170, a slightly better option http://www.airsoftgi.com/product_info.p ... ts_id=2355

Since you mentioned extremely high rates of fire, GBBR's are kind of out of the question since they are generally limited to about 1/10 of the ammo capacity that a high-cap mag has to offer with an AEG.

I have already built decent airsoft spring guns without breaking the bank.

Cost rises exponentially from building a spring gun to a fully automatic gun.

Frankly, I'll learn more from building it myself, I'll be able to fix it, and I'll be able to tune it.

I agree with you here, as this is also my ideology. But considering this is a gun for somebody else to be using, I'm not sure that the ability to fix it and tune it yourself is the best option. When she is out on the field and something goes wrong, no matter how major or minor, the user must know how to handle it. This won't happen with a store-bought gun, or at least very rarely and easier to handle.

Also, I can't justify a 200+ dollar expense.

Like I said earlier, it doesn't have to be that expensive.

Air is cheap. If I have to, I'll just go with a QEV semi auto, but I was researching my options. I can build a top grade QEV semi auto for around 50$, with a built-in hand pump for topping off the load.

That is an OK option, but in practice you will probably want something with a little better ROF and ammo capacity.
Electricity is even cheaper too. It doesn't cost much to recharge a battery pack, not to mention it is easier and can be done almost anywhere...

I built a very accurate airsoft sniper, with a bolt action loader and a high shot capacity. It just wasn't good enough to compete with an AEG firing hundreds of rounds at me.

I assume you're going for performance rivaling a good AEG for this project? If so, that will be extremely difficult. I know that in theory there are a lot of designs that should work reliably, but in practice, that is not so. With a homemade gun, something can always go wrong, and that risk increases when you don't have proper machine tools. When you buy an AEG or GBBR, it's going to be made with high tolerances and you get guaranteed reliability. You will simply never match that with a homemade airsoft rifle.

This is a special case. I would normally advocate for building the gun yourself. But since this needs to be especially reliable, I just have to recommend buying an AEG, just as an option. If you're hell bent on making it, I'm all for it. But think about the actual gameplay...

All your points are valid ones.
I am not looking for a rival to an AEG in performance, because I know that just isn't going to happen.
In the terms of user knowing how to use and fix, that actually isn't applicable. We play in our back pasture; when a gun breaks or stops, the whole game stops and they come to me to fix it anyway. I'm mostly looking to build a stop-gap, until I can afford to buy her a decent airsoft rifle.
You say that the 200$ price tag isn't true. Technically you are right. I did however, state "In my terms". As I said, to survive play with us, the gun has to be built like a tank. Even if a homemade gun doesn't perform like an AEG, I WILL be able to fix it no matter what happens to it. If an AEG breaks, there is no guarantee that it will EVER get fixed.
As I said, I'm just looking for some answers before getting started anyway. JSR has actually answered quite a few questions I had already.
Still, I see what you are saying.

P.S. It's nice to see someone else on Spudfiles advocating Airsoft Gi. They are my go-to airsoft guys.

As I said, I'm just looking for some answers before getting started anyway. JSR has actually answered quite a few questions I had already.

And I'd love to assist you in any way possible!!

Yeah, GI is my personal preference. They really have their act together, but I haven't bought anything airsoft in a long time. Actually never bought anything more than a spring pistol because I always preferred making my guns.

GBBR's are kind of out of the question since they are generally limited to about 1/10 of the ammo capacity that a high-cap mag has to offer with an AEG.

Not true anymore
For playing in the backyard, Qev plus pop off valve it's the simplest way for full auto, 50 psi will be more than enough.
If you're up to a challenge, check the second page of my topic (already linked by somebody else), that system worked and would be easy to convert to full auto.
Follow gun freaks advise. Airsoft guns are cheap in US, a Glock 17 GBB costs about 80$, tap a paintball QD on the mag, put a longer barrel on it then a stock. Voilà, semi/full auto rifle running on hpa.
Or a mini chambered BBMG, incorporate a hop up. The rof will be limited and more consistent shots.

I thought I posted this here yesterday but somehow it ended up in the wrong topic..


Anyway:


You could do a hammered pistonvalve in a traditional hammervalve configuration.
This could be fired with a crosman style hammer, with a relatively weak spring.. but will give a much more powerful shot.


By not sealing around the valvestem you could make the hammer blow back.The blast from the piston should be powerful enough, especially since you won;t need a very powerful hammer to set it off.



a. hammervalve and pistonvalve are closed. (REST)

b. hammervalve releases the pilotpressure.

c. pilotpressure has dropped and piston pops back ( FIRE)


( What do I know right? I've only got 1 reputation point... )

I'm confused as to how that is supposed to work.

It is a pistonvalve housed in a tube, set inside a larger reservoir.

It is piloted through the center of the piston via a hammer- or poppetvalve.

Since the hammervalve is orientated just like a traditional hammervalve it can be used in the same way.
The piston however adds a lot of power to the valve.

As it is triggered, the piston will slam back and open the airflow to the transferport, opening a much larger "hole" than you could achieve with a normal hammervalve.

It asks little, yet gives a lot.

Ah, okay, that makes sense. That's actually really clever.........you may have just answered my questions.

mako wrote:That's actually really clever...

I agree. 8)