Thank you for the description
Now I've got it

You mean I should screw the barrel to the blowgun?

But how to attach an O-Ring between blowgun and barrel? I want to load the gun though a hole covered by another pipe, then the O-Ring thing is not possible?!

The O-Ring is in front of the bb, right? So the bb does not get out too quickly?

Power: What you get out of a standard compressor, I think 8-10 bar
Size: Like a normal hunting gun, so it looks like real
Shots: 10-20 (but I can go with less if the chamber would be too big)
Money: Not more than 50 bucks for the metal parts

i thought I take the design here mentioned:
- 6 mm inner ...

Thank you for the links They helped me a lot!

Ehm do you know how much air goes out of a blowgun or how large the chamber has to be for a certain amount of shots?

So my idea of using an air pistol was right? But how much air/power comes out of such a pistol? Or is it adjustable?

Okay, now I've got another qquestion.
A ball valve lets all the air out, but for such a little bb you only need a small amount of air. And I don't want to fill it up after each shot.
How to achieve that?
With an air pistol like my father uses to blow the dust away?
Would that air be strong ang ...

Are epoxy conenctions that strong?

I've got another idea: I take a 10x2mm, 12x1mm and 15x1,5mm pipe (ebay). Then stick them into each other.

The inner is the barrel, the next will be for sealing and over that there is the outer pipe, like this:
http://img846.imageshack.us/i/unbenanntbk.jpg/

But how to attach them? Do they are such ...

I wanted to make a Single-Shot Pneumatic BB Gun and now I'm wondering how i could link the tiny barrel to a ball valve. It should be possible to load it without unscrewing the barrel, but with another tube sliding above the barrel to seal it.

Does anyone have a diagram for such a setup?

Greetings ...

@MRR Ok if I change the position of the ball valve with the fill valve, then the design should be like your "Big Boy" gun except from the size. Right?

We have copper pipe at home, but how did you attach it to the system. I am able to cut threads, but isn't it a weird size for my parts?

Thanks saefroch
Switching the ball valve and the shrader valve isn't such a problem, I will try it

The design is like this:
And I thought before I start with something own, I take a working design that is easy to build.

Pilot volume means, that the air which goes out to pull the piston will be to much? What do you mean, how can I solve this or is it ok for starting?

So the piston is the difficultiest part on the gun? But I will polish it properly with sandpaper and grease it, so there will be no problem.

@used ...