SpudFiles

I don't have any experience with pistons, and i was wondering if a tiny 12 cm 6mm barrel would have any usefulness (hitting targets at 40 meters) even at 70 psi or so. The chamber is a 3/4 in pipe, with a blowgun pilot.

So will it work ??

Not very useful. At 40 metres, you'd want a couple of feet of barrel length and higher pressures (250+ psi) in order to have a trajectory reasonable enough for aiming at those distances, as well as giving the projectile enough power to do some damage when it eventually gets there.

70psi through a 12cm barrel is little better than an airsoft spring pistol.

what JSR said + keep in mind that 6mm BBs are very light so they lose their energy very fast..... if you want to hit something at 40 m you should choose the heaviest ammo you can get...

And even a drop of wind will put the BB off target by metres. Spherical objects like to curve a lot unless you induce a spin on them to control them. The best way to do this is to add a hop-up to your barrel, it will make the BB raise in flight but it should shoot straight.