from day one with my two 300 RUMs, a Remington LSS and a Sendero, I have only used one powder, Retumbo, I have used two different Primers, the GM215M from Federal and the CCI 250 Mag, four kind of bullets, Barns TSX 180 gr., Berger 190, Berger 210, NAB 200 gr.
Both rifles have shot pretty good with just about any combination I throw at them. My best 3 shot 100 yards group was with TSX 180, my best 300 yards group was with Berger 210, the Nosler Accubond was mighty accurate at all ranges.
That's all my secrets...
I turned the necks on the Remington Brass and did not on my Nosler brass.
My final choice for long range in my Sendero is 93.0 grains of Retumbo with 210 Berger, Nosler brass, CCI 250, MV = 3023 ft/sec.
Hodgdon designed the Retumbo with the RUM in their minds, I'd bet, and they've
done a great job.