Another point of anecdotal data:

On the website http://castboolits.gunloads.com/, members there design a lot of moulds and put together custom orders. One of the more popular diameters for 6.5mm moulds is .268" on the driving bands, not for Carcanos, but for swedish mausers. Some moulds go all the way up to .270." There are a few that shoot them as cast at around .268" but most people size them to .266-267" to fit the swede rifles- but there are two different approaches to this, one is to use the largest bullet that can be fit in the throat and the other is to use a bullet that is at the groove diameter. I typically size the first 2 driving bands (on a loverin bullet) to .265" so that the base of the bullet is in the neck of the case and the bullet lightly engraves in the rifling. FWIW, most others are getting groove diameters in the .265-.266" range with a few actually having larger measurements.