As stated above, a chamber cast is necessary to determine the nominal caliber.I believe,however,that some of the above listed calibers can ba eliminated from consideration.The 6.5x65R is too recent to be considered.The 6.5x70R is a very slim (about 25-20 head dia.)cartridge used in small,light guns,and this BF has a 16 ga barrel next to the rifle barrel;the dimensions just do not demand such a slim caliber.The 6.5x58R is a good possibility,6.5x48R less so(due to utility).Two other good possibilities are 6.5x57R and 6.5x54(53)R Mannlicher. A good way to narrow the choices,absent chambercast,is by slugging the barrel.The 6.5x48/58/70R cartridges usually have a groove diameter of .260-.262";6.5x57/65R,usually .264-.266"; and 6.5x54(53)R is sometimes.267-268".
Mike
