I have seen Marlin 40/65 rifles in good+ condition going for $2500 to $3500; if you thinking of selling it I would certainly get it appraised. You could probably convert your rifle to any of the original calibers. "When introduced,the Model 1895 was chambered for the .38-56;.40-65,the same as the old .40-60 Marlin;.40-82;.45-70(including the various cartridges to include the .45-70-405 and .45-70-500 U.S.Government);and the .45-90,and its various loads. In 1897 the .40-70 WCF cartridge and in 1912 the .33 WCF cartridge were added to the Model 1895 list of calibers." The most obvious conversion would be to 45-70 with a barrel and bolt change, but it would destroy any collectors value. I could not imagine converting your rifle. If you really want a modern cartridge buy a rifle in 45-70, otherwise leave your 40-65 alone, it's simply too rare and valuable to mess with.