Hello.
I wander if I can turn down a "cold-hammered" barrel on a lathe, so it would get a smaller diameter?
This is original barrels from the Norwegian factory; Kongsberg. The barrel are the same as the one used in Mauser M59, and M67 (Sharpshooter-rifles).
I wander if I might disturb some of the tension in this barrels if I do?

What you need to know is whether those barrels were heat treated for stress relief after the hammer rifling process. If so, you shouldn't have much to worry about. If not, you may well see "springback" with removal of material around the outside of the barrel, enlarging the interior diameter.

Hopefully Bill Leeper, if he can drag himself away from my rifle for a minute, will chime in. He has had alot of experience in this regard.

Understand I can't comment directly as to what you can expect from this particular barrel since I have not turned one of these. I have, however, turned barrels from Remington, Winchester, Steyr, H-K, and Swiss Arms. All have turned well with no distortion apart from that which already existed. This last refers mostly to Remington barrels which are often far from straight. I have found hammered barrels to be usually quite inert. Regards, Bill.