The brand name escapes me, but there is a stump remover that is 90 something percent salt peter.... Works great! A simple way to make a burning powder from it is to just add sugar. The ratios escape me but I know 50 50 works. It burns very hot. No clue if its good for guns, but I used to use it for rocket fuel, called it rocket candy.
That is called Sodium Clorate, it is a potent weedkiller.
Did you intentionally reference "rocket boys"...
it is mentioned there.
Works better if you mix 30 sugar and 70 clorate.
According to Homer Hickman's rocket boys.
it would probably only work for a rifle since it burns, not detonates.
never have tried making things that can go boom in a firey way... Don't want to cross that line. Generally you get hurt eventually.