PDI Palsonite releases a 6.01mm inner barrel for the Tokyo Marui MEU, with an error margin of a mere 0.002mm per barrel.
When you want to squeeze a little bit more power out of your weapon, then a tightbore puts a better seal around the pellet to ensure a more stable flight path as well as a slight increase in power which doesn't hurt (well actually it can, you know what we mean).
The palsonite treatment adds a hard wearing surface with a lustrous black finish that has the added bonus of resisting the environment that much better besides also tidying up the aesthetics of you barrel profile by removing that unsightly stock brass finish from your muzzle.