Finally, a .243 Winchester for me & ...

[Catfish]

All of the powder manufacture have data on the web. Pick a load that will fill thr case to around 90% or alittle more and try it. The only way to know what is the best powder for your gun is to try all of them. But if you start with a powder that pretty well fills the case you should have no trouble finding a load that will do what you need done.