First of all I'd decide on the brand, and than look at what their options are.
Secondly, decide what brands of ammo you'll be using, and try to find their tolerances. You won't want a 6.01 barrel if you're wanting to use ammo that can have BB's that can be up 6.008 in diameter.
Unfortunately I won't be able to help you too much with barrels if you're going with the PDI hop-up, my experience with AEG type Barrels is limited to ones specifically for AEG's, different requirements.
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