I have 2f, 3f, and 4f powder all the same manufacturer.
If I load 30gr of 3f how can I determine how much 2f or 4f would be the equivalent charge. 4f would be lower amount of grains? 2f greater?
OK so you had some old information that's been taught for a while, mixed with what actually is going on....
First as the replies have stated, 4Fg is
most commonly used as priming powder in the pan of the flintlock. You will find some folks using it in very small caliber pistols, and using it to reload .22 LR rounds for end-of-the-world-disaster training, and such.
YES they did actually have granulation sizes for gunpowder [black powder] as early as the 18th century. So there was 1F, 2F, 3F and etc granulation system in place.
Today, when you see something like 3
Fg the little
g means that the granules are coated with graphite. This helps to keep them from breaking down into smaller bits, keeping "dust" down, and making them a teeny bit more resistant to moisture. NOW whether or not the frontiersman had a choice when he was at the trading post to refill his horn..., that's a different question.
You can, as folks have written, swap between the two powders by using the same powder "measure" which actually gives you the same
volume of either powder. Should not be a problem for the rifle or pistol.
NOW... I think..., my wife says this is dangerous for me..., you're asking about
the power of the powders vs each other? In other words, IF you use 80 grains of 2Fg, and you then use the same powder measure setting for 3Fg, what happens?
The old rule of thumb is you will get about a 10% increase in velocity. So you REDUCE the 3Fg charge by 10% when switching to that from 2Fg. (In this example you would subtract 10% which would be 8 grains, and round to the nearest setting on your measure, which would be 70 grains for 3Fg) That gives you usually, a very similar muzzle velocity for your projectile, and since most of us use fixed sights, it should put the projectile pretty much in the same spot on the target. At 50 yards you probably won't see much difference, and a deer won't know, but at 75+ yards, that change in a point of impact might mean the difference between dropping the deer in it's tracks (or winning a match) and you needing to spend a couple hours tracking the dying deer.
In the 30 grain example in your original question, going from 30 grains of 2Fg would mean setting your measure at 25 grains when using 3Fg
I hope my guess at what you were asking was right.
LD