According to Rifle magazine, Sept-Oct 2011 edition, and the article therein by John Barsness titled 'Mannlicher-Schoenauer 6,5x54 Carbine'; "Many older older 6,5x54 Mannlicher-Schoenauers... have oversized bores ranging from .266 to .269". The post-war rifles normally have .264" bores".

These above measurements are generalities, and slugging the bore of your rifle is the smart thing to do to be sure. The Hornady .264" 160-grain bullet gives good accuracy in my early carbine. My 1928 6,5x54 M.-Sch. carbine bore slugs at .268".