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Triple seven is a black powder substitute. Rather than trying to grind that, why not just buy Goex FFFFg Black Powder? You can get it from most major gun shops and it's very fine.Black Powder comes in coarseness grades designated by the number of Fs, from Fg to FFFFg. FFFFg (or 4 F) is primarily used for the flash pan on a flintlock. It's best to check local and state laws for use and possession of black powder, but judging from your invention above I'm guessing you aren't too worried about it. Hope that helps and keeps you a little safer.