The madbull black python version II (6.03) are very nice. I think that one of the last 4-5 that I bought was actually tighter than 6.03mm. They are aluminum and the inner barrel (of the inner barrel...
) has been treated to take out any/most "micro-roughness", resulting in a very smooth bore. I've only used one for a couple of games (est. ~2500 shots) and it seems to be holding up well. Because of the coating/plating process, it's not recommended that you cut these barrels to length.
The prometheus barrels are stainless steel and are very nicely finished. The last few that I installed seemed a little big (i.e. the outer diameter of the barrel). This made for a tight fit installing into hopup units and the brass ring, that typically centers/snugs it up in the hopup, would not go over the barrel. They have quite a ramped crown...so I'm not sure how hard that would be to replicate if you were to cut the barrel (SS is pretty tough stuff).
The System 6.04mm is old and tried and true. They're great barrels.
Best of luck.
Tys