I want a 45-70 pretty bad. I found a winchester 1886 with a LONG octogon barrel. I also found a marlin cowboy. The marlin had some nasty sights on it. If I had one I think I would do a short tang sight of some sort.
The marlin is 400.00 and the winchester is 1300.00 I would like to get the Winchester but I don't know about the price. About your question. The 300 gr should be ok for shots under 150 yards on big game. If I were to want to hunt elk with it I might go with a bullet in the 400 gr range.
Ron