6.5mm= .256"
I think you are mistaking 6.5mm for groove diameter, when it actually refers to the bore diameter in a swedish rifle. You are right about the other part of the dimension, most would have fallen in the range of 6.49-6.53mm, I personally haven't seen armorer's discs marked any smaller or larger.
If your rifles are measuring that large across the grooves, then they areoutside of the diameter they were when manufactured.
What were the specs for groove diameter? Any sources? I'm only giving you my first hand observations of the rifles that I own, and the work I've done with them. My newest Swede rifle was built about 64 years ago and my oldest 109 years ago, most of them no doubt have been rebuilt once or twice, but its tough to say what has happened since their last rebuild. All I can say is that I have no complaints about how they shoot with what I shoot them with.[8D]