I've probably loaded for more than a dozen .243's over the decades, and aside from the already mentioned 41-42 grains of IMR4350 (H4350 also works) with bullets in the 100-grain range, these have also done very well in more than one rifle, with both outstanding accuracy and velocity:
55-58 grain bullets: 48.0 grains of Big Game
70-grain bullets: 41.0 grains of Varget
Have also had excellent luck in recent years with 95-105 grain bullets using Vihatvuori N560 and Ramshot Hunter. Some rifles do very well with bullets over 100 grains with Retumbo, but the standard 1-10 twist doesn't do all that well with 105+ grain high-BC bullets in most conditions.
