Help with making a diy blowback system

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Airgunloverboy265
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Wed Apr 22, 2020 7:36 pm

Hi guy's I need some help. Im trying to make a fullauto replica of a micro uzi with a blowback engine and feeder. Essentially I want it to operate kinda like this commercial one but im trying to come up with an engine that uses no qev or electronic part's and would be powered by one of those compressor blowguns, what im aiming for is that when you pull the blowguns trigger the air would shoot through the tube and go into the engine and push the piston/ feeder back and forth like the actual gun and you can also cock it back as well. I tried using this design from the youtuber ukiliheal and followed the how to tutorial. while it worked in perspective it just went forward not back and forth. So does anyone have any suggestions or idea's on a homemade engine design that would work ?
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mark.f
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Thu Apr 23, 2020 9:25 am

Blowback is fairly simple to accomplish. Making it into something micro-uzi-sized is another story entirely. Take a look at here and here for some macro-scale versions.
P.S. welcome to spudfiles.
Airgunloverboy265
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Thu Apr 23, 2020 11:58 am

HMMM The second link might work I think i'll experiment on that one, also on the second link there is a pipe in the center of both the airtank and the barrel with the blowback, if i just put a straight tube going all the way to the blowback syatem would it still work? Also some of the images aren't showing on the first link you wouldn't happen to have managed to save them have you?
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mark.f
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Fri Apr 24, 2020 12:29 pm

Airgunloverboy265 wrote:
Thu Apr 23, 2020 11:58 am
also on the second link there is a pipe in the center of both the airtank and the barrel with the blowback, if i just put a straight tube going all the way to the blowback syatem would it still work?
I think you're talking about the chamber. :mrgreen: Kind of important for most valves we tend to use, but a hammer valve can bypass it.
Airgunloverboy265 wrote:
Thu Apr 23, 2020 11:58 am
Also some of the images aren't showing on the first link you wouldn't happen to have managed to save them have you?
No, mainly just linked it so you could see the video.
Airgunloverboy265
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Fri Apr 24, 2020 11:58 pm

You couldnt find the images? That's ok I still appreciate the links you gave I'll definately experiment with the designs a bit. Also I came across this blowback design today viewtopic.php?f=26&t=20754&p=283271&hil ... on#p283271 you think it would work for a fullauto blowback as well?
Airgunloverboy265
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Sun Apr 26, 2020 5:19 pm

Also just a little side question if i use one of those mini pumps used for bikes how many pumps would I need to do to reach 12 bars?
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mrfoo
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Mon Apr 27, 2020 1:18 am

Boyle's Law tells us that, for a constant temperature, pressure is inversely related to volume, so for a fixed quantity of gas, if we reduce the volume by a factor of 2, we raise the pressure by a factor of 2.

So, if we assume you have a 100mL chamber at atmospheric pressure, and you want to raise its pressure to 12bar, you either need to reduce its volume 12-fold, or put 11 times more gas into it (effecitvely, pretend it had 12 times teh volume and reduce it to its existing size) - you'd need to pump 1100mL through your bike pump.

So to calculate N, the number of pump strokes it's going to take, you need to know :

Chamber volume C, volume of air pumped per stroke P

N ~= (C x 11) / P.

This ignores the effects of temperature rise, which will be non-negligible.
Airgunloverboy265
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Tue Apr 28, 2020 12:32 pm

Here's a quick sketch at what I'm aiming for. I also went ahead and drew the custom airtank( modeled after a commercial one) that I made out of a soda bottle ( the large ones), a ball valve , an air tube, and a threaded end with an o ring attached for the Blowgun. So essentially its like how I described, when the air shoots out the Blowgun the blowback engine would go back and forth like the actual one. So anyways anyone got tips for making it work?
Also thanks for the bike pump answer😉
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