Oh, thank everyone for the answers!

So... the pipe diameter is 2"; its thickness is 3mm... so the external is 2.12".
About the type... i really don't know, i bought it in an hardware store and it the same kind of iron used for my house's pipes... however, the blow test is beyond my possibilities cause i don't have a similar compressor (and as you said, i think it's too much dangerous!).
My question was about a general type of iron, just to have an idea... and brian said that a normal working pressure is 25 bar... so i think i can try a 4x mix for example, right? (I've read somewhere that 4x mix produces 18-20 bar... so i think it would be fine!)
About my project... i took a photo with the digital camera, but i can't find the usb cable in this disordered room... but when i'll find it, i'll be glad to send some photos!

About the project... all has start with one idea: making a backpackable hybrid potato gun. So barrel is detachable (obviously) and divisible in many parts, like the fuel meter part.
The chamber can be easily replaced with another one (for future upgrades!) and the barrel is modular, so i can put a silencer (to be made) on it, just screwing it at the top! ^^
About the ignition, i think i'll try with a camera flash (cause here in italy the stun gun is illegal and is really more cheap) powering a spark plug (an automotive one).
Thank everyone for the answers... and if it will works, i'll write something on the wiki as promised
