I can sum up your problem in one word, pyrodex.
The ignition temperature of pyrodex is 800 deg, whereas the ignition temperature of black powder is 400 deg. In addition, black powder is not nearly so corrosive to the gun and your measuring tools as pyrodex. It is also less hydroscopic.
The list of advantages to real black powder is long. It’s list of disadvantages is short. Namely hard to get. Black powder is classified as an explosive (because of it’s low flash temperature), and so dealers have to store it in a case instead of displaying it on a shelf. Call around and I am sure you will find it at a good dealer.