Micro-Groove barrels shoot cast well, just takes a little more work. Load the largest diameter bullet that will chamber without difficulty. In the .45-70, this will usually be .460" or even .461". As for hardness, former gunwriter and cast bullet expert C.E. Harris recommends a hard bullet for Micro-Groove barrels. I've had success with soft bullets of wheelweight alloy, but I never shot them faster than about 1,300 fps. To me,the Marlin is a hard kicker if loaded much beyond that velocity.
Many say that these barrels won't shoot cast bullets well, but I think they got discouraged when they didn't get good results with poor fitting bullets or bullets of the wrong alloy.
