Rule of thumb (I have this printed out in my Black Powder reloading book as I load for small revolvers and large rifles)
Granulation:
Black powder is produced in different sizes or granulations as indicated by the number of F’s on the container’s label, with Fg being the coarsest and FFFFg being the finest. The size of black powder you use depends on your firearm and the manufacturer’s recommendations.
Fg: Coarse grain typically used in cannons, muskets, rifles larger than .75 caliber, and shotguns that are 10-gauge or larger.
FFg: Medium grain typically used in larger rifles between .50 and .75 caliber, 20-gauge to 12-gauge shotguns, and pistols larger than .50 caliber.
FFFg: Fine grain typically used in smaller rifles and pistols under .50 caliber and smaller shotguns.
FFFFg: Extra-fine grain used as a priming powder in the pans of flintlocks. Never use FFFFg black powder as the primary powder charge in a rifle, pistol, or shotgun.
Substitutes:
Two of the many black powder substitutes are Pyrodex and Triple Seven. Both are available in granulated and pellet form. Use Pyrodex in volume equal to black powder; but when using Triple Seven, reduce your loads by 15%.
Pyrodex offers three different granulated black powder substitutes. Pyrodex P (for pistols) is the same size as FFFg black powder; RS (for rifles/shotguns) and Select (a variation of RS) are like FFg powder.
Granulated Triple Seven comes in FFg and FFFg sizes.
Loading procedures with substitutes may vary. Be sure to get loading instructions from a qualified gunsmith and your owner’s manual. Substitutes are not recommended for use in flintlock priming pans because they may not ignite from sparks as easily.