The only barrel that is 6.03mm I could recommend with any good conscience would be the Prometheus. Otherwise, other 6.03mm brands just aren't as accurate as wider bore barrels in the 6.05-6.10mm range.
I run PDI 6.05mm and 6.08mm barrels in all my my rifles, save for one, which has a Prom. 300mm 6.03mm, but thats a stubby M4 used in CQB.
A link to a good technical review of barrels done in Germany, for Madbull. While I usually don't like to quote sources whose potential to be biased is obvious, they do show that Madbull barrels, even ones hand picked for the test, don't do as well as compared to PDI (X-fire in Europe), or Prometheus in terms of roundness or precision.
http://www.madbullairsoft.com/images/Advertising/MadBull%20Barrel%20Report.pdfHere is a small translation I did for the barrels I found relevant:
9. X-Fire (PDI)
Declared Value 6,01 mm (according to the manufacturer)
Absolute diameter 6,021 mm
Deviation of manufacture= 0.011
Change of inside diameter 0,002 mm
Material: Steel
Overall Rating 83%
Tuning potential 88%
The manufacturer X-Fire is known here in Germany is still under PDI, but calls himself for quite some time X-Fire.
The myth of the barrel, many have been waiting for is now unveiled.
A really good run, but again, the manufacturer promises a little more than it can deliver. Interesting when you look at
that Prometheus can even do a bit better. In terms of precision this is the best in this test run. But also significantly more expensive than the DBC runs!
10. Prometheus
Declared Value 6,03 mm (according to the manufacturer)
Absolute diameter 6,036 mm
Deviation of manufacturer = 0.006
Change of inside diameter 0,001 mm
Material: Steel
Total score = 75%
Tuning potential 68%
Prometheus makes all the other manufacturers see what is round.
The barrel has extremely small difference of only one micron speaks volumes. The barrel is shows the precision in terms of production that it ranks with the best that there is! A good tuning run, which only slightly worse is defined as the X-fire operation. Thoroughly recommended!
According to available information, the Nine Ball runs are identical to the Prometheus barrels. It is money well spent.
20. Madbull V2
Declared Value 6,03 mm
Absolute diameter = 6.016 mm
Deviation from manufacturer -0.014 (If interpreted as a deviation from zero)
Change of inside diameter = 0.004 mm
Material: Aluminum
Total score = 88%
Tuning Potential 85%
Again, the reader can see from the deviation from the manufacturer "that this course has something special."
This barrel is clearly very tight, as specified by the manufacturer, which is why two additional specimens were tested to make sure that this does not distort the rating to positive outliers. The Madbull V2 barrel is actually the tightest currently available on the market, that there is.
The accuracy in the roundness of other manufacturers, however, it does not reach. X-Fire (PDI) and Prometheus demonstrate against what is technically feasible! Madbull v2 barrels are highly recommended for the money offered a very good choice and a smaller restriction. The smallest diameter in the entire test, the barrels tend to be susceptible to the use of non-quality ammunition.
Also, Aluminum barrels wear very poorly especially in Arizona, where dust and grit combine with the friction forces of the BB being thrust down the barrel to erode the barrels inner surface rapidly.