Dunno how many .243's I've loaded for, but so far either IMR4350 or H4350 has shot well with 100-grain bullets in every one, whether it has an 8, 9, or 10 twist. Generally around 41 grains does the job.
Me too, works in every 243 and 244 I've ever had so I've never gone further with load development with other powders.
