[Ricochet]

Bore size alone does not affect accuracy or range (technically), although too-tight could be an issue. The tighter it is the more effect you'll get from dust and grease in your barrel on the BB. Tightness will have an effect on velocity, tighter being higher vs lower on the same power source. What I mean is, if you don't change the spring size, say an M130 spring, will have varying velocities due to bore width. In fact wider bore barrels, like 6.08 plus can show significant velocity drop and may require a larger spring or power source to get the BB down range nicely. A heavier spring is tougher on your gun from drawing more battery to cycle, giving you less shots, to outing more tension on your internals overall and making your motor work harder.

All that being said, stack up three guns with the exact same hop, BBs and velocity, but with different note-width on their barrels and technically your shots should be the same or similar. They probably won't be, but in theory. So for all of the previously said reasons my opinion is strongly that 6.03-6.04 is the best range of barrel, no tighter and no wider. You don't have to worry about fouling and cleaning as much, or even often to be honest, and you don't need to enhance your spring or gas flow to comensate for power loss. Your focuses should be: barrel quality, BB weight and quality, and most of all hop-up.

As an aside; back when standard guns came with 6.08 barrels, PTWs came with 6.04 and most sniper projects ended up with 6.03 barrels. These are still popular setups today if that tells you anything.