Peep sight, I think that your '81 Marlin may be chambered for the 40-60 Marlin cartridge. This cartridge is dimensionally the same as the 40-65WCF.The base and rim diameter are the same as the 45-70. I have a Colt lightning Express rifle chambered for the same round. Just run 45-70 brass through the 40-65 sizing die, trim to 2.105" length and you're done. Starline makes excellent brass, also, for this cartridge. It can be purchased at a very reasonable cost from MidwayUSA.Have a smith do a chamber cast to be sure, though.
