With all due respect ... it may not be the barrels ...
FWIW we have a mess of Zolis (8 or more>) at our BP club and they ALL shoot well, as in REALLY well! One member completes at the No-So Skirmish Assoc. Nationals and has a set of plug gauges for 58-cal bores. We have found the Zolis run to the larger size, running from 559 to 583+. We used cast lead minies sized (the tool by Lodgewood) to be 0.002" under whatever the bore gauges to and the results are great!
In my 2 Zolis, one uses 578s and the other I use the as cast at around 580. I find them VERY sensitive to the size of the projectile. We do all of our load development at 50-yards until we get a 3~4-grain spread maximum where the groups are one ragged hole (benched) or close to it. Then we go out to 100-yards and start load development in 1/2-grain increments at the bottom of the spread and work up to 'fine tune' the accuracy load.
Once tuned in, both shoot 1-hole ragged hole groups at 50-yards!