George

Nearly every ZKK I've ever seen has a dodgy crown, and benefits greatly from a re-crowning job... depends on whether there are also rust pits at the crown or not. Even just a ballbearing hotmelt glued to a suitable length of tube and used with valve grinding paste to recut the crown at right angles to the bore will work fine if there is no pitting.

Another problem with .308 ZKK 601's (and I have two of 'em) is that the chamber is often very large towards the rear (excess runout when machining?), causing 'potbellied' fired cases that won't fit back into the chamber at all unless precisely indexed to where they were fired

Moving the barrel in one extra turn and rechambering is another relatively cheap fix that often restores accuracy without losing the unique ZKK barrel profile with integral rear sight base.
A standard barrel profile just doesn't look right IMO, and machining in that double dovetailed sight base is VERY expensive... if you can find a gunsmith even willing to tackle the job.

I'll try and post some pics if I can find them of my reworked ZKK600 in .270... with 6x32 Zeiss Jena QD scope. Yes, a 6x scope was a very rare factory option vs the usual 4x QD scope.

A simple quick'n'dirty ball-bearing recrown halved group sizes at 100m to ~30mm with Remington Corelokt factory 130gr loads... I was going to rebarrel it to 9,3x62mm but it shot so well for a hunting rifle I kept it as a .270

Cheers

TM