I,ve got the NRA caliber specific loadbook for 243 Winchester and IMR powder shows astarting load of 44gr up to a max load of 46gr of IMR4350 powder at 42,900-48,600 cup pressures. That is for 75 gr bullet. It lists the velocity at 3127-3380 fps, don't know the length of the test barrel though. They do mention that a faster burning powder such as IMR 4064 did perform better with the lighter weight bullets. Good luck.