i've reloaded the 6.5mm Carcano round for over 50 years. The Carcano bore is .268 in diameter. Most Carcano rifles will not shoot well using the .264 bullets that are widely available on the US market. Hornady does make a 160 grain .268 bullet that shoots really well in my Carcano rifles.
Years ago i came into several thousand rounds of 6.5mm Carcano ammo that was made in Italy during WWII. In my rifles that ammo is as accurate as any other WWII milsurp ammo.