The 243 is one of those rounds that has a reputation for being a little finicky.
I stuck a strain gauge on my 243, and settled on 44.5 grains of the old H4831 behind a 100 grain Partition or Solid Base for mine. That gives me 2960 FPS at a measured 54.7 KPSI.
The old H4831 is less dense than the SC version, and just nicely fills the case.
