Question regarding piston for mini coaxial

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PatchyThePirate
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Tue Feb 23, 2021 11:38 am

I am in the process of building my first pneumatic air gun which is a mini coaxial. I have made the piston out of a short length of pen cap which has been filled with hot glue and has a piece of 3mm rubber as the sealing face. There is no visible gap but It slides easily down the tube. It also won’t move on its own when I plug one end of the chamber with my finger. Is it too loose?
The brass barrel has already been epoxied in place and I have tested the piston by putting it in the chamber followed by the bike tire valve wrapped in tape to hold it in place. It seals after two pumps in quick succession with a small shock pump.
This is my first time building a pneumatic spud gun and I don’t really know how tight the piston should fit.
Pictures attached of piston fit.
Thanks for the help.
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jackssmirkingrevenge
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Wed Feb 24, 2021 10:25 am

Welcome to the forum!
PatchyThePirate wrote:
Tue Feb 23, 2021 11:38 am
There is no visible gap but It slides easily down the tube. It also won’t move on its own when I plug one end of the chamber with my finger. Is it too loose?
Image

Looks just right to me.

One thing though is that the hot glue construction might not survive higher pressures, remember that apart from the compressive forces there is also one heck of an impact every time you fire.
hectmarr wrote:You have to make many weapons, because this field is long and short life
PatchyThePirate
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Wed Feb 24, 2021 3:39 pm

jackssmirkingrevenge wrote:
Wed Feb 24, 2021 10:25 am

One thing though is that the hot glue construction might not survive higher pressures, remember that apart from the compressive forces there is also one heck of an impact every time you fire.
What is the highest pressure hot glue can be used at? My piston has a diameter of around 9mm and the barrel 3mm. The I plan on using it in the 250 to 350 psi range. If I am using the right formula (3.14159* .35 * .35 = 0.384 in^2. 350 PSI * 0.384 in^2 = 134 lbf) 134 pounds if my math is right. Would hot glue work under that kind of force if the plastic tube it is in is roughed up well? Or should I look into using epoxy or a solid rubber one? But that would require me to make an entirely new piston. The epoxy I have right now is kinda thick so to make a piston it probably needs to be thinned down. Thanks for the help.
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jackssmirkingrevenge
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Thu Feb 25, 2021 10:04 am

PatchyThePirate wrote:
Wed Feb 24, 2021 3:39 pm
Or should I look into using epoxy or a solid rubber one? But that would require me to make an entirely new piston. The epoxy I have right now is kinda thick so to make a piston it probably needs to be thinned down. Thanks for the help.
I've had success with a rubber piston even at 850 psi.

Easy to cut from a sheet of thick rubber by using a sharpened tube, typically the same material as the chamber.

Watch your fingers though!

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pneumatic marker project thread
hectmarr wrote:You have to make many weapons, because this field is long and short life
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