Expansion ratio of the 243 lends itself to slow powders, 4350 has been my go to, as it always worked well and I typically had some on hand - IMR. Used others since. I do like Varget pretty well for the light 70s but anything like 4350 or slower works great with 85 to 100+.
Given your accuracy - not sure you will improve but that never stopped me from experimenting with powders and bullets - in the name of "R&D"
