I am not sure if accurate 150-grain loads are a myth or not. I see them on TV (in ads for rifles & Power Belts), but not at the range. I have seen some good groups shot with 100 grain pellets of T7 and SST's in a sabot, but no good 150-grain groups.

I have had excellent results shooting a 370-grain Maxi Ball in a .50 with 140 grains of ClearShot FFFg which gave a velocity of 1740 FPS @ 10' from the muzzle, but ClearShot is nowhere near as powerful as any of the other ML propellants!