SpudFiles

I've been looking at BTB's air ram full auto, some paintball deigns and the QDV (quick dump valve) design, found HERE.

I started thinking, and came up with this blowback design. I could make this easily ( Have a Milling machine), but would have to invest in a 80$ rotary table.

It would fit in the same sort of barrel assembly as a QDV, but would move forward, to be pushed back by the air pressure, recock the spring, lather rinse repeat.

Worth a shot, do you reckon? Have a look at the Images.

EDIT= The shape of it means that there is a greater surface area pushing back than the just the cross section of the pipe, would that be enough to over come the difference in friction between the bolt and the projectile?

Is there a way you can make a diagram displaying how you expect it to work?

i do agree with hubb more info is needed, a simple paint animation would be sufficient

In theory that sounds like it would work, in practise i am afraid that unless the pressures you are using are very high and the projectiles heavy and the barrel long, it won't work.

I am sure you can work it out with some formulas, finding out how much energy it would take to push the piston back(remember that O-rings will add to friction, maybe a member can supply a few plausible figures on how much friction they cause) and to recock the spring(if you know much much energy it takes to push back the piston you can find a spring that matches this). Going from there you work out either:

How heavy your projectile must be and how long your barrel must be, when you have a fixed pressure
or
How what pressure you need if you use a fixed mass for your projectile and a fixed barrel length.

Perhaps with an extremely light piston, a lot of lubrication, heavy projectiles, a long barrel and high pressures you could get it to work, good luck with it.

If you decide to work out if this works please show us the formulas and calculations you did, a functioning blowback would be a very interesting design(IIRC we had a blow back design for the last contest however no videos were posted of it in action) especially one that is this 'simple'.

Essentially what I was aiming for here.



What I found in practice was that there was too much friction to blow the bolt back, there are ways however of increasing the available area for the air to act on and therefore increase the blowback force on the bolt.

Friction is what I was worried about, any chance that with an extra O-ring to tighten the projectile in the barrel this would work? or would it not fire at all?
Like this, see where the extra red is.

wow pretty clever... combination of the QDV, the hammer valve and the pure blow-back

personally I think it would be better if the front part of the piston was just a simple tube (that would offer more flow than just a couple of holes drilled in it).

EDIT
I don't think that you need an oring there (instead just choose a tight fitting ammo)...
I think that friction won't be a problem... it all depends on the force acting on it (so either you need to use a large diam bolt/spool or high pressures)

go for it

Or you could do something similar to what BTB is doing and use a gas piston.

the reason why al-xg used a gas piston and a locking bolt is because he felt there is too much force acting on the bolt... so the bolt is allowed to move only when ammo is right at the muzzle...

That sounds positive, and aluminum can hold shiteloads of pressure, so if I make it high pressure would be fine.

Alternatively, but I would much rather use the other design, what about this

Xerxys wrote:what about this

The balance of pressures seems to indicate it won't blow back at all...

it seems a bit more complicated which is not always a good thing

also ammo should be loaded when the valve is closed
here it is loaded when it's open... I don't know how much time there would be for ammo to drop into the barrel

The balance of pressures seems to indicate it won't blow back at all

naah it will... the both sides of the part with holes drilled in it will balance itself out... so there is still one surface on the bolt exposed to the pressure (the one directly at the air source in port)

I think I'll stick with the simple blowback, the aluminum tube I would like to use;
1" OD 1/8"wall yields at 14000psi, so a safety factor of 4 still leaves me at 3500psi, which is a lot.
Very much a lot.
Ear protection, machine gun power a lot.

I have know way of achieving that pressure though, other than co2 which would be ridiculous. Any ideas?

Scuba tanks would be a good source of high pressure.

my guesstimate is that it will work even with a 0.5" ID barrel (or even slightly smaller) and pressure as low as 200 - 300 psi

Have a look at al-xg's gun and BTB's pnushnikov... as I mentioned earlier

al-xg used a gas piston and a locking bolt because he felt there is too much force acting on the bolt...

so it might not require as large sizes and pressures as you think