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Unless you are using a Cannon with a 6 pound ball and a three pound charge, then it really is more economical to buy what you need.
While Black Powder, (also Red Powder and White Powder) can be made, unless you are quite familiar with the process, it is not something to be piddling around with. If you have to ask the question on how to make it, this includes you!
It is also a process that can take days, and it's not like whipping up a batch of stuff in the kitchen. Then you really have to test it, using the proper method or equipment to see what you have, the burning rate, etc.
Quality control is also a factor, along with safety conditions. If you screw up, you can exit by the large hole that suddenly appears in the roof of your house.
Commercial Black Powder is granulated for size, and has a fairly consistent burning rate, and has been tested for loads and charges. You can find information for loads for various calibres in many reloading books, but with an unknown powder of an unknown burning rate, then you are asking for trouble.
If you think Black Powder is expensive, what value do you attach to parts of your body? Too many things can go wrong if you screw up.
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