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Blowback questions / help please

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Blowback questions / help please

Unread postAuthor: Patto » Wed Nov 17, 2010 9:43 pm

Hey all,

I've just come back after a while off and am now ready start up again. And for as long as i can think i've wanted to build a blow back system. So I’ve had a good look around and done a lot of reading but want to run a few questions past everyone..

JSR posted a few pics of different setups on this Page (BB feed method) and this is the one that has caught my eye

Image
Thanks jack.

So i went home last night and grabbed 1 of my bolt action guns just to test the theory.

So my set up isn't quite the same as the above as the air does not pass up through the bolt and out, but rather the bolt holds the BB in front of the exhaust hole. Anyway I loaded a BB but did not lock the bolt. As I fired the gun the BB shoot and the bolt opened. :lol: things are looking good.

The one problem I had was when I used a spring or even a rubber band I could not get the bolt to cycle back far enough to load the next bb. I thought about the bolt length etc and made a quick prototype with a closer load to exhaust hole which did work better, but still had flaws. So before I go any further I would like to ask some questions..

1) Has anybody actually built this design?? If so have you had any luck with it ?
2) Do you think it's vital to have the air passing through the bolt, rather than in front like mine is now
3) If so, as this is / will be 6mm will there be restricting issues ?
4) And if I was to O-Ring the bolt either side of the exhaust hole , would the friction be to much & would I have to worry about chopping the O-Ring’s ?

I see this as a great idea, but at the moment I’ve run out of material to make a prototype and I would just really like Everyone’s ideas and opinions first.

Thanks
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Unread postAuthor: Crna Legija » Wed Nov 17, 2010 9:59 pm

what pressure are you using this would really need 250+, jack made one that is simular and works but it uses cartridge.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 17, 2010 10:08 pm

What's vital for it to work is a large enough "pressure pulse" in the breech to give the bolt sufficient impulse to cycle.

There are several ways to make this happen:

- longer barrel

- heavier projectile

- higher chamber pressure

- o-ring/seal detent to restrict the projectile

- tighter projectile (for example 0.25" pellet in a 6mm barrel)

As -_- pointed out, it is vaguely similar in concept to my "valveless cartridge" design, of which I proposed a non-cartridge version, read through page 6 of the thread ;)

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Unread postAuthor: Patto » Thu Nov 18, 2010 10:03 pm

-_- wrote:what pressure are you using this would really need 250+

At the moment im only running at 100-120psi for testing , although I have the capabilities to run higher , but I would like to see if I could get this to work at 100psi since not everyone has a shock pump or fridge comp.

@ JSR- I had a good read through your post, thanks

At the moment im running around a 500-600mm barrel using .25" steel bb's (don't know what weight though) and since I’ve move the load port and exhaust port closer together (shorter bolt travel length) it's basically working :lol: , it's just not consonantly loading the next BB. I guess this could have lots to do with the mag set up, as I know the bolt retracts far enough I just don’t think there’s enough time for a bb to load (from a hopper setup - without a force/spring behind it)before the bolt wants to move forward again ? if that make any sense?

Thanks for all the info man
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 11:47 pm

Basically the same issues I was dealing with, is there that all-important o-ring or detent keeping the BB at the breech?
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Unread postAuthor: Patto » Fri Nov 19, 2010 12:25 am

No there’s no detent or O-Ring at the breach at all. Put the barrel is a tight fit and i don't ever have a BB roll down the barrel ?

One thing i keep thinking about with that is aren’t you worried that if you create too much friction holding the bb, wont the bolt open (releasing pressure )before the bb can get going and lose performance ?

.... oh unless the mag/hopper is sealed so there air pressure has no where else to go ??? is this where this is going ?
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 19, 2010 12:56 am

Patto wrote:One thing i keep thinking about with that is aren’t you worried that if you create too much friction holding the bb, wont the bolt open (releasing pressure )before the bb can get going and lose performance?


This is where the importance of an adjustable bolt spring comes in, it needs to be tight enough to allow enough pressure build-up for the BB to "pop" through the o-ring, yet not so tight as to fail to recoil far enough to pick up a fresh BB. Again this is completely analogous to my cartridge prototype.

An airtight magazine would certainly help, but if you time it that the BB has left the barrel by the time the bolt exposes the mag, it's not necessary.

I'm tempted to build this actually, might put one together this weekend, if only as a "demonstrator" for low pressure and airsoft BBs.

Edit: started construction of a low pressure airsoft calibre blowback prototype as per the above design concept, I'm hoping that if it works the lower specifications will make it more accesible to the average forum member (and perhaps further my epoxy crusade :D) and also I'm casting in clear tubes so the mechanism can be seen internally, this seemed to make the blow forward breech quite a hit :)

Here's the basic breech before pouring the epoxy, you can see the 6mm steel rod (from a printer cartridge, great cheap source) that's serving as a mandrel, keeping the barrel (on the left) and bolt tube (on the right) in line. and the breech and bolt o-rings in place, as well as the 1/8" threaded section to which the blowgun will be attached. I decided to drill out the magazine hole after the cast is finished as opposed to incorporating it in the mould.
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Unread postAuthor: Patto » Mon Nov 22, 2010 2:03 am

Looking good, I like the idea of the clear casing.. Helps everyone with being able to see it at work, rather than just trying to understand :) I cant wait to see how this turns out.

This is where the importance of an adjustable bolt spring comes in, it needs to be tight enough to allow enough pressure build-up for the BB to "pop" through the o-ring, yet not so tight as to fail to recoil far enough to pick up a fresh BB.


But with having the barrel o-ring your going to need some good pressure to squeeze through that o-ring(Good Power), but you will have to adjust the bolt spring up harder as well to compensate, but then wont this stuff you up as your still going to lose pressure as soon as the bolt opens the magazine port? and as the bolt spring has been tighten up, the turn around time of the bolt will be a "lot" quicker than that of a weaker set spring or no o-rings ??
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 22, 2010 2:09 am

Patto wrote:Looking good, I like the idea of the clear casing.. Helps everyone with being able to see it at work, rather than just trying to understand :) I cant wait to see how this turns out.


I had a play with it over the weekend, there's no way it's going to work at shop compressure pressures as it is. What I want to try is eliminate the bolt seal, it will mean some air loss but at least the friction will be reduced to the point where it has a chance of cycling.

But with having the barrel o-ring your going to need some good pressure to squeeze through that o-ring(Good Power), but you will have to adjust the bolt spring up harder as well to compensate, but then wont this stuff you up as your still going to lose pressure as soon as the bolt opens the magazine port? and as the bolt spring has been tighten up, the turn around time of the bolt will be a "lot" quicker than that of a weaker set spring or no o-rings ??


I see what you mean but it's down to imparting momentum to the bolt, if you look at my valveless cartridge videos you can see that the bolt continues to fly back even after it's no longer being pushed by the cartridge, in this case one hopes the bolt will keep on going past the magazine port even after the air is lost.
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Unread postAuthor: Patto » Mon Nov 22, 2010 6:09 am

in this case one hopes the bolt will keep on going past the magazine port even after the air is lost.


Thats what i was thinking, i guess this is where the high pressure benefits this application as the "Pulse" of air would have enough power/momentum to push the bolt all the way back past the mag port.

i've played with this set up with 1/4" brass telescoping inside 9/32" (so no seals but perfect fit) and can get it to load the next bb 2/5 times @110psi with just a rubber band as the return spring for the bolt. but i don't know where to go from here but up the pressure ?
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 22, 2010 7:18 am

I would definitely look into the possibility of adding an o-ring in order to increase the pressure "pulse".

Another more complex possibility is adding an external gas piston type assist for the bolt, maybe something like this (ignore the cartridge system)

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Unread postAuthor: Patto » Mon Nov 22, 2010 7:41 am

i've thought about this design before but doesn't everyone say that your going to need high pressure for this system to work ?
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Unread postAuthor: POLAND_SPUD » Mon Nov 22, 2010 7:56 am

well yeah but not if you use a detent... ohh and I think you should build a direct blowback first
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 22, 2010 8:06 am

Patto wrote:i've thought about this design before but doesn't everyone say that your going to need high pressure for this system to work ?


I used to think that, but it's exactly how the FX Monsoon works. Granted it uses a minimum of 1500 psi, but on the other hand the bolt and hammer spring is extremely hard to cock, so with a big enough piston you should be fine.
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