I have some Pyrodex RS powder & wondered if that was the FFg or FFFg? Or is it different? And can I shoot it in my inline ML? I read somewhere that it is meant for percussion cap ML's.

Pyrodex RS means Pyrodex Rifle & Shotgun powder. Yes you can shoot it in your inline rifle and it will work great. It is considered FFg grade. For a percussion cap & ball revolver they suggest Pyrodex P which is for Pistol.