The designations of fg, ffg, fffg, ffffg so on and so forth, is determined by the size of the grains of powder that will pass through a mesh screen of a specific ‘Finess”; fg is coarse, large grain powder…ffffg is very fine, small grain powder. The coarse powders have a smaller surface area, with a corresponding slower burn rate, finer powders have a higher surface area with a faster burn rate.
Larger calibres have heavier masses of projectiles, smaller calibres have lower masses of projectiles. Think of these projectiles as being doors…one door is heavy and made of steel, the other door is a light hollow core door.
If I walk up to the hollow core door, I can easily kick that door closed with my foot. The door is light and easily closed; similar to using a quick burning powder to launch a light small calibre projectile.
If I walk up to the heavy steel door and try to kick it closed, I’m libel to break my foot. I need to apply a slow steady pressure to push the door closed; similar to using a coarse slow burning powder to launch a heavy projectile.
With a muzzleloader, you want to accelerate the projectile in such a fashion that the soft lead of the bullet (ball) will upset…meaning that the back of the bullet accelerates faster than the front, causing the soft lead to upset or flow into the rifling, allowing the rifling to spin the projectile, while the lead & patch combination form a gas seal.
Using fine fast burning powder behind a heavy projectile causes a pressure spike to occur. So when we use fffg in say a muzzleloader of .54 calibre or larger we feel more recoil when shooting the gun, as there is a high pressure being generated behind the heavy ball. It is theorized that over time, repeatedly shooting heavy loads of fine grain fast burning powders behind large heavy projectiles will cause the barrel to eventually fail, due to metal fatigue from the constant high pressure spikes
Using coarse slow burning powder behind a light projectile may not have a burn rate fast enough to cause the lead to upset into the rifling, and the projectile is blown out of the barrel, rather than being shot out of the barrel. Accuracy is negatively impacted, and velocity is reduced; because the projectile never upsets enough to properly engage the rifling and gases produced, just blow past the projectile; because there is no effective gas seal.
ffg is great for calibres of .50 or higher…and smoothbores, which are also usually greater than .50 calibre.
fffg is great for calibres of .50 or smaller….50 calibre seems to be that division where it can swing both ways.
ffffg is a very fine fast burning powder, used as a priming powder in flintlocks.