Sorry, didn't see this!
The "which bore is best" debate is almost everlasting.
The consensus seems to be that 6.03 is the best, 6.01 is held in disregard because people say they get jammed up because obviously, they're a tighter fit around the BB.
But then there's the "expensive" upgrade parts companies, that tend to market 6.08 as their most accurate barrels and there's also barrels you can get which are rifled.
I use a 6.01 in my L85 and I've used 6.03 and the rifled "twist" barrels too. 6.01 gives by far the best performance. Greater range and accuracy than the others if all other things are equal.
The reports of them jamming seem to be a load of bollocks, I've never had a BB jam with mine and I've never cleaned it or anything. Just use good quality BBs that have been polished... Which practically all have apart from those misshapen shite ones from Skegness that are obviously dodgy lol.
Only downside can also be considered an upside. Due to the tighter bore, they up your fps by around 30, which can take you over your site limit in some cases.
But on the flip side, this means you can use a spring less powerful, which means you'll get a better rate of fire, a quicker trigger response and it'll also increase the life of your gun's internals by a small fraction due to the lower strain of the spring meaning your motor and gears don't have to work as hard to turn the mechanism over.
As for the barrel brand, Madbull barrels are perfectly fine and reasonably priced. The Black Python V2 6.03s seem to pop up everywhere and everyone and their mums have got them fitted in their guns.
Mine is a Madbull V1 455mm long 6.01 diameter one though. I would have a 509mm, but I initially bought the barrel for an M4 project and it ended up being about an inch too long, so I tried it in the L85 and decided to keep it in there. Only about £30 too.