Fox Airsoft does a comparison of 6.01mm and 6.03mm Tight Bore Barrels in this video... "You'll remember in a previous video we talked about upgrade barrels and what they do. If you decide now that you need one, the next step is choosing what kind. Do you want a 6.01mm or a 6.03mm? Do you want it to be aluminum, brass, stainless steel or steel? What's better? We'll give you some food for thought in this video."