Here's my notes:

Bore slugged by Christian Neubauer, gunsmith 03/10/11. He reports:

Mannlicher:
.2564 .2565 .2564 across three different locations on the lands
.2681 .2680 .2678 “ groove
All measurements done in thousandths of an inch with a 0—0001” reduced anvil micrometer. Note, reduced anvil means small anvil, about 1mm.

Ray Ordorica discusses bore sizes in Handloader's Digest 15th edition. In essence, groove diameter of pre-1945 6,5x54 M.Sch. rifles is typically .2666 to .268”. Greener, quoted by Ordorica, says lands .256” and grooves .268”. Bingo ! So 4 thou over bullet size is normal for pre-1945.

Just keep in mind that, the oversize Carcano bullet you refer to is not very tapered, and will jam in the lands (and create real pressure issues), if set to a normal C.O.L. found in reloading manuals for 6,5x54 M-S. To establish the correct C.O.L. for your rifle with this bullet you'll need a special tool that was made by Stoney Point but which is now retailed by Hornady.