When I bought my 300RUM I had heard alot about berger bullets, Retumbo and Ramshot Magnum powder. I figured I didn't want to shoot anything lighter then a 180gr bullet and didn't want to shoot anything heavier then a 210gr bullet. So I tried 185gr, 190gr and 210gr bergers. I was very disappointed with the groups I was getting. So I played around with seating a little farther out, they will still fit in the magazine, have not shot those yet and I am also going to try a few that are longer the the magazine, those will be an experiment as I don't want a single shot rifle.
Then I came across a thread about the Hornady 208gr Amax and Hodgdon US 869 powder. I have not found much data on the 208gr Amax. So far this 300RUM Sendero has been a hard gun to get good groups with.