My new project. Will eventually be my first pistol build. Picked up a blemished Spikes Tactical "punisher" lower for a steal. Has two screw ups on it. No biggie. I think someone started a build and buggered it up, then returned it saying it was out of the box bad. Oh well, I got it dirt cheap. DMPS LPK and Mil spec upper. Being this is my first pistol build, what size barrel should I look at? piston driven system? Or just stay with a low profile gas block w/ tube? Will have more questions coming.