the chamber is 1/2" pvc and the barrel is... well a bic pen tube.
Operating pressure At the moment is 150 psi when i get around to it ill test it higher and possibly get better damage also.
Damage at 150 psi is minimal, barely penetrating 10 sheets of printer paper at 2 feet away with copper bbs, Which don't fit very good maybe if i find a better fitting ammo ill get better damage also.
It has a diaphragm not a piston. The diaphragm is just smaller then a dime.
im going to add an 1/8" ball valve instead of the push connect fitting but i want to take it to work and all the airlines connect via push connect..
I'd post a video but like i said nothing really to see. after i get the pressure up maybe to 200 or so and find a better piece of ammo i may post a video. but here is a picture

EDIT: tested to 200 psi, with airsoft bbs which fit perfect, damage is alot better, now if i can find some metal 6mm bbs.