Since you have an 1886 as opposed to a Marlin you have another great option. The Speer 350gr Magtip is a superb bullet for heavy 45/70 loads. It was designed for the 458 Win and works well in the 45/70 when stoked to the gills. It’s too long to function in a Marlin when crimped in the cannelure but works fine in my 1886 Extra Light. Fill the case up with H4198 and let her rip.

This buck from Kodiak island caught one in the throat from 75 yards. It pulverized a vertebrae and left a 2” exit in the back of his neck. [Linked Image from i.postimg.cc]