Re: 300RUM help
If you have your bullet choice narrowed down, which it sounds like you do, the next step is to get a couple or few different powders that people have had the best luck with. Usually you can tell which powders these are. With the 300 RUM they tend to be H1000, Retumbo, or RL25 with a cople others in the mix.
I shot nothing but H1000 in my 300 RUM with the exception of a few with Retumbo. My rifle Preferred H1000 from the get go so I had no need to experiment with different powders.
Primer choice is usually one of the last things to change. They are not that important in developing an accurate load, BUT a different primer can make all the difference in the world, cutting some groups in half from my experience. 9 of 10 times though the primer you have on hand will work for load work up. If you get the hankering though, keep everything else the same and do a simple primer swap and see what happens.
I would recomend you try some Retumbo or RL25. Every rifle is different and you cant expect them all to shoot the first powder/bullet combo you throw at them into 1/2".