SpudFiles

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 diameter barrel with epoxy in a reducer
- air pistol with epoxy in a reducer
- both reducers connected
- air pistol connected with a tube to a chamber (a pipe with two end caps)
Materials will be aluminium for the barrel, iron for the chamber and brass fot the reducers and connecters. Air filling will be solved through quick disconnects.

Is this plan okay?

I think this design by lozz08 is a perfect design for you to base your project on.

Of particular importance is the o-ring between the "air pistol" (blowgun) and the barrel, because this holds the BB enough for pressure to build up behind it, as well as lowering rate of fire to an extend which makes semi-auto practical.

Also note that he uses a relatively small metal chamber but filled with a shock pump to around 25 bar, giving him a good number of shots.

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

Exactly. A blowgun is not a perfect valve, but keeping it in the breech until pressure builds up improves performance significantly.

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?!

Sandwich that way, the more you tighten the fittings, the more the o-ring will hold the BBs

You mean I should screw the barrel to the blowgun?

You get two fittings, like a cap and nipple. Drill a hole in the cap the size of your barrel and glue it in place. Then sandwich a peace of rubber with a hole a tiny bit smaller then your ammo in between the fittings. You can adjust the pressure needed to pop the bb out by tightening/loosening the fittings. Connect the back half of the nipple to you blowgun.

Thank you for the description
Now I've got it