The "F" size refers, as you have mentioned, to the size of the gunpowder granules. The smaller the size, the more overall area when fired, so the faster the burn rate. So you are correct. (It stems from a very old system of screens used to sift the finished powder.)
The different sizes were generally 1Fg is cannon, 2fg is muskets and some rifles, 3Fg is pistols and smaller caliber rifles, and 4Fg and smaller is for priming flintlocks and for use in some very very small pistols.
The slower the burn rate, the lower the pressure. Now for some folks they want to shoot heavy loads, perhaps maximum loads. Such loads might be safe for say an original Brown Bess musket, or a reproduction of the same musket, and they would most often use 2Fg powder in that musket as it would be between .75 - .80 caliber, depending on who made it and when. Many folks switch to 2Fg for .58 caliber (24 gauge) and larger bored rifles and guns. Some folks switch to 2Fg with 28 gauge or .54 caliber rifles or guns, but I use 3Fg for my .54 rifle. I don't use anything near a max load, so the rule of thumb isn't set in stone.
However, IF you used a max load of say 120 grains of 2Fg in a 12 gauge caplock shotgun, and switched to the same load using 4Fg, you could easily load much more actual powder as you'd eliminate the tiny spaces between the granules when using the smaller sized powder. Black powder is measured for loading using grains of powder by
volume while modern powder in a smokeless shotshell is loaded with grains of modern powder by
weight. They are not interchangable systems of measuring, and the powders are also not interchangable.
So when you used the same powder measure, with the same setting, but switched to 4Fg..., you'd have actually added more powder, probably beyond the max load.
And..., since you used 4Fg, when it fired, the much faster burn rate could far exceed the max pressure the barrel can handle, perhaps well beyond tested pressure (called "proof") of the gun.., and you'd get a burst breech or barrel.
So that's it in a nutshell.
You're 2Fg will work in a regular cap-n-ball revolver. Most of the Colt and Remington reproduction revolvers in .36 or .44 use from 10 to 18 grains of 3Fg..., but you can use the recommended powder loading in your manual and use 2Fg.
Something like a .44 Walker or .44 1st Model Dragoon often like 2Fg. A smooth bore .62 or larger "horse pistol" akin to that used by G. Washington or later by Wellington's cavalry would use 20 grains or more of 2Fg. Something like this
NAA .22 cap-n-ball revolver uses something like 2 grains of 4Fg.
I hope this helps.
LD
