Well, I don't shoot .243, but I DO shoot Rem 6mm (virtually the same beast in all ways that would matter to you. H380 is unrivaled--it's the only powder that rifle gets fed. I use it for 1000-meter shooting mainly with 90gr Sierra FMJBT's, but it fires the Speer 105's for hunting at 1/2 MOA all day long on H380 also.
"Why is is called Common Sense when it seems so few actually possess it?"
Guns only have two enemies: Rust and Politicians.