Mav - heresay, aka internet rumor, as well as my old man, etc... heard in a gunstore type story.
Guy gets ahold of a rechambered arisaka. Chamber is .30-06, barrel is still 6.5mm. Supposedly fired it several times and thought it had way too much kick. Sent said gun into gunsmith, who in shock, sends it off to the NRA. Supposedly the NRA confirmed that this actually happened. Supposedly Arisaka was unharmed and still in usable condition.
Can you imagine the chamber / overall peak pressure of a .308 bullet being shoved down a .257 ish barrel???? IF this story is true, and it should be verifyable if it was the NRA that tested it, then that action should be able to hold up to any pressure you throw at it. .300RUM only has a max pressure of 65k. That .30-06 was probably in the 80+K range if this really happened. I believe that the NRA does say it is safe to rechamber/barrel these to '06, but I would check with them on that. Sounds like a good time to send an email to the NRA.