having a 6.03 won't necessarily increase your accuracy, it's the consistency of the bore, and compression behind the BB that increase accuracy by stabilizing the BB.
Case and point, stock GBBR's with 6.08 barrels are usually REALLY accurate. And I've seen some really shit quality 6.04mm barrels first hand, they actually reduced accuracy from the stock barrel.
If you have an inconsistent bore, it slows the air around the BB at random, and inhibits laminar flow behind the BB. Having a perfectly smooth barrel allows for laminar flow of air, and ideal compression around the BB to help it stay centered in the barrel.