6.01mm diameter inner barrels leave so little space between the BB and the barrel that you really need to pay attention to the ammo you use.
A tighter gap between BB and barrel does make a better seal but a seal that is too tight can disrupt air flow and cause irregular flightpath travel. A wider gap allows smoother movement but also leaks more pressure and can cause a drop in power and power inconsistency from one shot to the next.
To optimize, you need to match the right ammo with the right barrel. In the case of a 6.01mm, you need to look are premium quality ammunition and do your research to pick out smaller BBs as slightly brands can and often do get stuck in 6.01mm barrels if not at least choke.
Matching your gun to the right barrel is both a science and an art. You can use the barrel designed for it or you can apply shorter or longer versions in special custom ways to get modified results.