I have the same gun. All the Marlin 45/70s Ive owned liked .460 dia bullets so thats the diameter i tried first. I was pleasantly surprised to find out my 86 shoots amazingly well with that diameter bullet. I got a 350 grain Ranch Dog mold from NOE and it is a great bullet that functions well through the 86 action, and shoots good to boot. Most of the newer 86s have short throats, and some cast bullets wont work, so just be aware of that.
