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Join Date: Jan 2010
Location: Southcentral PA
A .25 cal bullet will fit snugly into most 6.5 caliber barrels. So, if a .25 cal bullet can be dropped into the barrel or pushed in (not hammered in) and a 6.5 bullet cannot, it is likely a 6.5 barrel. If the .25 cal bullet does not drop/push in without much difficulty, it is likely a .25 barrel. With no markings on the barrel, I'm guessing it was rebarreled so you are not likely to find an answer via serial number.
I think slugging the barrel and a chamber cast are the safest options for determining what you have.
I'm not sure what you are calling "close" in terms of chambering a 25-06 cartridge. Neither a .257 Roberts or 6.5 Swede chamber should come anywhere near being able to close the bolt on a .26-06 cartridge.