Powder and bullet weights are calibrated in grains. The unit was based on the weight of a single grain of barley. There are precisely 7,000 grains per avoirdupois pound. Powder is measured by volume so it is a good idea to check the calibration. Note that double F and triple F will weigh different amounts for the same volume. It is interesting that gunpowder, bullets and arrowheads are almost the last remaining use of this ancient unit of measurement although some pharmaceuticals still mention it.
Black powder granulations are determined by a sieving method. The holes in the first screen are .0689” square and the holes in the second are .0582 square. Any granules small enough to pass through the first screen and too big to pass through the second screen are Fg and so on. FFg doesn’t pass .0376, FFFg doesn’t pass .0170, FFFFg doesn’t pass .0111.
You may see the term "dram equivalent" on a box of modern shot shells. It refers to a charge of FFg black powder in early shot shells and gives an approximation of comparitive power.
A dram is one 256th of a pound. Easier to remember is 16 drams equal 1 ounce and 16 ounces equal 1 pound. You will never need to use it today unless you are researching in an old book.