"Fine rifle powder" would be nearer fffg powder. Generally, Fg was the coarsest, cannon powder for field artillery. FFg was next in line and used for muskets and shotguns. FFFg for smaller bore rifles and pistols. FFFFg is priming powder.
Now, for my observations. It takes longer to burn through coarser grains and in my .577 PH 1853 pattern Enfield, if I shot any heavier than 60grs in 2/fg I tended to have burning powder blow out the barrel. That makes for inconsistent and less accurate burn rates. Another problem was that the 2/fg left coarser fouling. With 3/fg I got better accuracy and cleaner (well, not so dirty) bores. May be why the Brits called for "Fine rifle powder."
Consider, they did not issue a "cleaner bullet as did the US army, (1 per every 10 rds.) and did not use a canelured bullet either but did not seem to have any particular problems with excessive fouling build up. However, in practice, the Enfield bullet was loaded with the lubed paper wrap.
However, just as an aside, I have read one particular case of a badly fouled bore. A Texas Confederate of the Texas Brigade wrote how in "Devil's Den" at Gettysburg his rifle had become so fouled he was finally reduced to beating his ramrod end against a rock face and in that rifle's final shot, the thing pulled out of his hands whacking a fellow behind him bringing the soldier a solid cursing out. He then picked up another musket and continued his work.
Finally, a loose rule of thumb of the day suggested that the powder charge be equal to the caliber of the barrel rounded to the nearest 10grs and that brings us back to 60grs for the .58/.577 musket.
"In times of trouble and of war, God and soldiers we adore; When wars are done and wrongs are righted, God's forgotten and the soldier slighted." Ye Olde English proverb; origin unknown
I make reproduction Yugoslav pre-WWII rifle slings and bayonet frogs.