My ABS 300 RUM 11.25x HATES Retumbo, likes H1000, H50BMG (with the 208's-220's), RL-25 and very little else.
Beware that you most likely will NOT want to use the same charges in your barrel as you commonly hear about. I hear about 93+ grains under 200-210 bullets alot. In the last 2 custom 300 RUM barrels I have owned including my ABS barrel I can get 92.0 MAXIMUM for a 180 grain variety and depending on the bullet it is 90.0. H1000 with the 200 AB wont do squat for me with any charge. However, H1000 under the 208 AMAX is awesome. So is H50BMG with the 208 and 220 SMK. The 178-180 likes 90.0 of it. RL-25 has worked with all the bullets I have tried from 180-200 grains. Again, 90.0 grains is max for the 180's and 86-87 is max for the 200 AB.\
Youre rifle will be different. Try several powders.
NOTE: Both my 12x and 11.25x barrels like 178's and 180's with 90.0 of RL-25 @ 3300 FPS. And 86.0 RL-25 @ 3100-3150 under the 200 AB. These maybe "pet" loads????? 2 barrels liking the same loads? Sounds like a sweet spot.