The new modern custom barrels have the rifling cut after the barrel has been bored. My understanding is they cut it like you would cut a thread with a lathe. Hence they can cut any rifling twist and even varying rate of twist in the barrel.
I have seen chatter first hand in a buttoned rifled just at the muzzle, it didnt make any difference in accuracy that I could notice at the time.
Just to round out the thread I ended up with an ATA 308 in Walnut
$915 (plus $150 for property letter, $240 for WA license application but I got cz515 and sulan tac-12 on same application )
Now as much as I want a K98k in 308 they go for twice as much and shoot far worse which isn’t a big deal - if I put it on collectors licence in WA I can’t ever shoot it.
If I put it on open license I can probably justify it to WAPOL saying it has iron sights and the ATA has a scope but it won’t be half as useful as the sub moa Turkish gun.