Fg, FFg, FFFg are used to describe

Black powder is made and sold in the form of irregular shiny metallic-looking black grains designated as to the size by the letters FG, FFG, FFFG, etc. The more the number ofFs, the finer are the grains. One gram of powder produces 3000-4500 cc of gas.Ref: Textbook of Forensic Medicine and Toxicology; 5th ed; Page no: 238