Lancer Tactical 6.03mm Precision Barrel. I'm wondering why nobody bought this before. $15.75 for a stainless steel, durable, sturdy tightbore barrel? The thing weighs ten times as much as a G&G stock barrel, and you can feel the quality in the heft. Immediately after installing the barrel, my gun shot much farther, the FPS rose and stabilized at a consistent 340 FPS (Airsoft GI G4 9" MID), and the accuracy was spot-on. It works so well with the G&G green hopup bucking, especially when using nice and heavy bbs such as .25s and .28s. I seriously recommend this barrel.
Stainless Steel (NO bends or cracks anywhere, nor I don't think it's really possible unless you smash it with a hammer!)
Increase in accuracy
Increase and stabilization in FPS
Huge increase in range due to better air seal around the bb
Cheap as can be, yet still retaining so much quality.
None, it's a great barrel. It did come to me in a little bit of a dirty condition (particles in the barrel), but of course I cleaned it and it wasn't a hassle whatsoever. Everyone should clean their barrel once they've bought it, even though it's brand new.