The recommendations that Shawn gave are spot on so I have nothing to add there. What I would like to add is that you may want to consider that the reason the lighter bullets are not shooting as well may be due to the round you are shooting them out of at least in part.
Let me explain. You have a round with a 100 gr case capacity. In my opinion, the 300 RUM should be loaded with at least 180 gr bullets to get the most consistant results with this size of a case capacity. In fact, the heavier the bullets, generally the easier it is to fine a quality consistant load with these big rounds. The 180s are good, the 200 even better.
Just seems the heavy bullets allow the large case capacity to work much more consistantly compared to lighter bullets.
Also, remember that most 150 to 168 gr bullets are not in any way designed for the velocity potential of the 300 RUM. When most were designed the biggest round out there was the 300 Wby and to be honest, most were used and tested in rounds like the 308 and 30-06.
As such, they are not generally designed for the velocity potential of the big 300 RUM and that may also result in some accuracy issues.
Again, not trying to tell you which way to go with your project, but I will say on averge, it is much easier to find a truely consistant and accurate long range load using a heavy for caliber bullet with these large case capacities. Plus you get the advantage of better ballistic and terminal performance as well.
In some cases light bullets work very well though so it greatly depends on what you want to do with the rifle.
Good Shooting, just some things to think about! /ubbthreads/images/graemlins/grin.gif
Kirby Allen(50)