Precision polished 6.03 tightbore inner barrel for PTW / Action SL-MK4 AEG. Enhances accuracy and power. With an inner diameter of 6.03mm it is a tighter fit on ammo then most stock varieties (which are in the 6.05+ range mostly) but still not as tight as 02 or 01 barrels making them easier to work with. 6.03mm is generally considered the sweet spot for combat weapon as its tight enough to achieve some performance improvement yet loose enough to accept common ammo types.
Increases muzzle velocity
For the Action SL-MK4