The way to rememer it is;
Pyrodex RS is FFg grade. RS stands for rifle and shotgun.

Pyrodex P is their FFFg grade and the P stands for Pistol.

Fg grade is commonly cannon powder.

FFFFg grade is a priming powder.

P is 15% stronger then RS.

Many people shoot FFFg in their rifles. The reason they claim is, it burns cleaner and produces better pressures and less fowling. I personally shoot FFFg in my sidelocks and inlines when they are .50 caliber and under. All you have to remember is to reduce the load approximately 15% from what you would shoot with FFg. Actually the easy way to remember using Goex FFFg is consider it
Triple Se7en FFg. As your aware, in loose form, Triple Se7en is 15% stronger then Pyrodex RS or Goex FFg.

People that shoot flintlock rifles almost always shoot Goex or other brands of Black Powder because of the low ignition of the substance. I shoot Goex in all my rifles. Besides the cost, I like the way it shoots and cleaning the muzzleloader was never an issue with me.