I agree. I use IMR-4895 for my 85 grs. When shooting 100's, I have found that IMR-4350 works great! If a .243 won't shoot with one of these powders, taking into account bullet weight, I'd be surprised. These two powders seem like they were made for this caliber.Welcome to ShootersForum, Lowndleft.
The 3rd edition of the Sierra manual shows the following loads for 243 with their 100gr bullets:
IMR4895 -- 31.5 starting charge : 36.2 maximum charge
H380 -- 32.8 starting charge : 36.4 maximum charge
Both powders fall short of best velocity as they are a little too fast for this cartridge capacity and bullet weight combination. They would be better with lighter bullets. Pick a flavor of 4350, 4831, or maybe RL19 for better performance.