Compact Semiauto Spud Gun

Miniature guns are novelty custom, unique, and sometime downright crude! Common construction materials often include pill bottles or pens. Show us your work!
ExtremeGamerYT
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Wed Jan 18, 2023 5:22 pm

Hello Everyone, I have brought a compact design I have been thinking on for over a month, I tried making the fastest firing possible (without multiple combustion chambers) on an alcohol gun, the design relies on a blowback bolt, so if the combustion does not have enough pressure I think it won't be possible sadly, and will probably be needed a piston to push the bolt automatically.
The neat part in this design is the pipe in the middle of the bolt that carries the combustion gases from the combustion chamber, while also serving as support for the spring and bolt (the combustion chamber also serves as the right hand handle, just a bit bigger than usual), which will press the ball forward and the bolt backwards, automatically cycling the magazine of the gun.
the scale and placements of parts are subject to change, and the outer parts/case are not drawn.
The drawing has some explanations about each part, orange and red parts are meant as cylinder walls cutaways.
Please tell me what is wrong in this design, and how big do I need the combustion chamber to be to have enough power to pull the bolt backwards automatically.
This is my first post, so I'm sorry if the image comes out wrong. :D
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ExtremeGamerYT
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Thu Jan 19, 2023 11:31 am

I forgot to mention the valve between the alcohol tank and the combustion chamber, which is manually opened and closed.
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jackssmirkingrevenge
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Fri Jan 20, 2023 1:55 pm

I don't have any experience with launchers using alcohol vapor but they seem to work well enough and I suppose it will generate enough pressure to work a simple direct blowback bolt as drawn. What's less clear is how you intend to get an ideal fuel/air mix in the combustion chamber, and of course it's important to make sure that the flame can't flow back to the main fuel chamber.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Fri Jan 20, 2023 11:58 pm

jackssmirkingrevenge wrote:
Fri Jan 20, 2023 1:55 pm
I don't have any experience with launchers using alcohol vapor but they seem to work well enough and I suppose it will generate enough pressure to work a simple direct blowback bolt as drawn. What's less clear is how you intend to get an ideal fuel/air mix in the combustion chamber, and of course it's important to make sure that the flame can't flow back to the main fuel chamber.
Thanks for the feedback :D
I don't think it will have a good fuel to air mix, because it uses the alcohol vapor generated by bubbles on the alcohol tank by the air pump, which can be a very poor mixture, so I would most likely inject every 30 firings 10ml of alcohol on the combustion chamber just so there's enough fuel already in it, but it isn't ideal.
To make sure that the flame doesn't go back there will be a one way valve and maybe a flame arrester, but I don't think the flame arrester is necessary, as the flame isn't extremely powerful
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jackssmirkingrevenge
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Wed Jan 25, 2023 11:38 am

You might want to check out this launcher and other posts by user Rambo who did quite a few experiment with this sort of system:

Image

In this case he was actually using the alcohol reservoir as the firing chamber, the premise being that only the vapors would ignite while the liquid remains.
hectmarr wrote:You have to make many weapons, because this field is long and short life
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