This is all Plagiarized from various sources, but here goes:

F = "Fine", g = "granulation"

The "F" designation is just the screen size used in manufacture and the resultant grain size (coarseness).

Here is one recomendation:

There are four (4) types of black powder which are designated by granulation.

• 1f or fg - used with cannons and large gauge shotguns.
• 2f or ffg - the most commonly used powder in muzzleloading rifles, shotguns and pistols of .45 caliber and larger.
• 3f or fffg - used in muzzleloading rifles, pistols and revolvers of .44 caliber and below.
• 4f or ffffg - used to prime the pan of a flintlock

But, 100 grains of black powder is not equal to 100 grains of pyrodex is not equal to 100 grains of tripple 7. So you have to do your homework (read the accompanying documentation).

As I understand it the more F's in the formula the finer the granulation of the powder. Therefore FFFG is "finer" than FFG. If I understand this correctly it means that you'll get more granules of powder in the same volume of space which means that you'll get a faster burn/more bang from FFFG than FFG.

Another search lead me to this:

"FFFFg is the smallest popularly employed grain; it burns fast and was used primarily in handguns. FFFFg is considered flintlock pan powder. FFFg and FFg were a bit larger; they have long been used in rifles and shotguns. Fg was the largest: used in very large bore rifles."

You got my curiosity up on this one. I had never studied the topic before. So this is just a compilation based of a quick scan on the internet.

Good luck!