Your barrel stamp photo shows 7,8mm and 57. I believe that 7.8mm rifles use .318 bullets as previously suggested and not .323 bullets. Someone on here I am sure will own a rifle marked in this way and can give you the definitive answer. Otherwise, you could get a gunsmith to cast the chamber and give you correct bore/groove and bullet measurements. Suggest you just buy a simple micrometer or caliper to check bullet diameters on factory loads - about $35 for peace of mind. Or if you are going to stick with the same brand ammo, just phone them up and check what they are loading.