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why didn't it work?

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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why didn't it work?

Unread postAuthor: hysten » Sat Sep 26, 2009 11:40 am

Hi, this is my first post on spudfiles and i am super excited! I was hoping to get some feedback on my newest bb gun project.

Essentially my rig works as follows:

-a 6mm ID brass pipe (12in long) from a cleaned out weed sprayer is being used as the barrel

-inside the barrel there is a piston that is blocked on one end with epoxy (piston is 4in long)

-there is a hole drilled though the side of the piston to allow a 1mm OD nail to fit though the piston

- on the both sides of the barrel there are two slots that are 2 in long and 2mm high

- then 2 rubber bands are attached to ether end of the nail that is put though the piston

-the idea is that the air pushes the piston forward and the rubber bands pull back the piston

-yesterday i ran the rig at 45psi and the performance sucked, the bb (12g 6mm) hardly went 10 feet

Am I doing something wrong? Or do i just need to take a different approach?
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Re: why didn't it work?

Unread postAuthor: spudtyrrant » Sat Sep 26, 2009 12:02 pm

draw of a diagram or take a picture and we will be able to tell you more and what you can do to make performance better :wink:
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Unread postAuthor: hysten » Sat Sep 26, 2009 12:04 pm

i tried to host one on imageshack, but it never showed up :cry:
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Unread postAuthor: spudtyrrant » Sat Sep 26, 2009 12:05 pm

hysten wrote:i tried to host one on imageshack, but it never showed up :cry:
than draw a quick diagram on paint
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Re: why didn't it work?

Unread postAuthor: Technician1002 » Sat Sep 26, 2009 12:23 pm

hysten wrote:Hi, this is my first post on spudfiles and i am super excited! I was hoping to get some feedback on my newest bb gun project.

Essentially my rig works as follows:

-a 6mm ID brass pipe (12in long) from a cleaned out weed sprayer is being used as the barrel

-inside the barrel there is a piston that is blocked on one end with epoxy (piston is 4in long)

-there is a hole drilled though the side of the piston to allow a 1mm OD nail to fit though the piston

- on the both sides of the barrel there are two slots that are 2 in long and 2mm high

- then 2 rubber bands are attached to ether end of the nail that is put though the piston

-the idea is that the air pushes the piston forward and the rubber bands pull back the piston

-yesterday i ran the rig at 45psi and the performance sucked, the bb (12g 6mm) hardly went 10 feet

Am I doing something wrong? Or do i just need to take a different approach?


The biggest thing I can think of is the piston is 4 inches long. It would weigh much more than the bb, so when the entire arrangement accelerates the vast majority of the kinetic energy is used to accelerate the piston. Next on the list is the rubber bands attached to the nail through the piston. I the nail goes through sideways, to two rubber bands can attach to both ends of the nail. The total piston travel distance is only a couple inches.

As a result of high mass, low force, and short acceleration distance, the results are low velocity.

I think you need to take a different approach.
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Unread postAuthor: hysten » Sat Sep 26, 2009 12:40 pm

here is a diagram
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Unread postAuthor: jeepkahn » Sat Sep 26, 2009 12:44 pm

myself and JSR are both fans of using springs to push the piston closed after firing, as long as the springs light enough it won't effect performance much/ifatall...

edit: I think you've got the wrong idea about piston launcher, you've basically built a pneumatic ramset gun...
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Last edited by jeepkahn on Sat Sep 26, 2009 12:47 pm, edited 1 time in total.
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Unread postAuthor: hysten » Sat Sep 26, 2009 12:45 pm

Technician1002, thanks for the advice

I think you are right in the fact that the piston is way to long. I orginaly put the rubber bands on the end of the nail, but i guess i didn't communicate that very well.

Here is my new design
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Unread postAuthor: spudtyrrant » Sat Sep 26, 2009 12:53 pm

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Unread postAuthor: Technician1002 » Sat Sep 26, 2009 1:00 pm

hysten wrote:Technician1002, thanks for the advice

I think you are right in the fact that the piston is way to long. I orginaly put the rubber bands on the end of the nail, but i guess i didn't communicate that very well.

Here is my new design


The other problem I see with the modified design is once the piston stops, there is no air pressure behind the bb, so the rest of the time in the barrel it is trying to pull a vacuum behind it. Performance may improve if the barrel is simply cut off at the end of piston travel or air holes are provided along the barrel to use the barrel for a projectile guide. There is no further acceleration in the barrel after the piston stops.
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Unread postAuthor: jackssmirkingrevenge » Sat Sep 26, 2009 2:10 pm

Am I doing something wrong? Or do i just need to take a different approach?


You're using a 12 inch barrel at 45 psi, performance isn't going to be spectacular.

Here's what's possible with 0.12g BBs pushed by 300-400 psi in a barrel 3 times the length of yours ;)
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