Almost forgot, my last M96 6.5x55 had a .266" groove diameter -- it's bore was brand new, appears unfired, as-did the bolt.
That rifle put 120 Speer, 129 Hornady, 140 Hornady and 160gr. Hornady's into 1 1/2" at 100 meters off bags using the battle sights and Reloader #19 as well as Hodgdon H450 powders.