Let me check my reloading data......I sold my 45-70 marlin, but that was all I used was IMR 4198......
your barnes orginial 300 gr, that jacketed or cast?
ok, I found Seirra 300 gr FN, starting load of 43.2 gr, vel: 1747, Max load: 48 gr vel: 1901
so anywhere between them two you should be...