If you can find a copy of Handloader's Digest 1996, there's an article by Ray Ordorica about his 6,5x54 Mannlicher-Schoenauer. He slugged his pre-war Mannlicher and discovered it to be .269" (which I think he acknowledges some of which might be wear). Most 6,5mm bullets of modern manufacture are .264". However Ray points out that, in Greener's "The Gun and it's Development", the bore of the 6,5x54 M. Sch., (which the English call the .256" Mannlicher-Schoenauer), has a groove depth shown as .0059" for an approximate total groove diameter of .268". Frank de Haas in Bolt Action Rifles also indicates the groove diameter of 6,5 Mannlicher's is .266 to .268". In other words, these (presumably all pre-war rifles) were all within original specs.

After the war, the Austrians began manufacture of this rifle in 1950 with the American market primarily in mind and made a number of changes to better suit the American taste, such as the swept-back bolt handle, side safety etc. My guess is that their change to .264" occured then to better suit their new market. Because this was an adaptive thing, with there being a Model 1950, Model 1952 and also intermediate types, as well as Model GK for the home market, then later types still such as Model MCA, you really cannot assume anything without slugging the bore.

I'm able to use .268" Hornady "Carcano" bullets for very good accuracy and velocity without pressure factors, because I've done my homework and slugged the bore - twice! If I loaded to SAAMI overall cartridge length specs, for instance, I'd get into all sorts of trouble with this large diameter bullet, which is probably why Hornady advise, "Carcano only". That's why I recommended to the forum instead the Hornady .264" 160-grain bullet which is much safer if you have no way of measuring distance to lands and/or also don't want to slug the bore.