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Go with 14x1 if 9/16 x18 or x24 is not available . 9/16 is 14.29mm.
14x1 is one of the standard threads of ase ultra. There should be enough meat on both barrels to do that thread from memory.
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Actually the sako/ tikka stainless barrels are pretty easy to turn down and get a nice finish thread onto.
The old hammer forged chrome moly BSA / parler hale and old Remingtons can be a bit more...

This one :
Muzzle Threading — Don’t Remove Too Much Steel « Daily Bulletin

First , sako barrels are hammer forged not button rifled.
Measure the outside diameter of your barrel where the...