Hi RegimentalPyro.
Another way of making black powder would be to mix all the ingredients as you have done grind them aswell if they are quite corse,And then add just boiled water,Mix to make the whole mixture into a paste and so it is all wet,Then leave to dry once dry grind as you normaly would,Doing this should cause the potassium nitrate to disolve in the water,Then as the water evaporates it will form crystals of potassium nitrate onto the atoms of sulphur and charcole which should make the whole mixture burn quicker as the atoms have there own supply of oxygen from the saltpetre.
in theory.
Hope this helps.
That's the method I found. What I found suugests that after you dissolve the nitrate and add on the C and S, add in some isopropanol to help ensure that the nitrate precipitates out of the solution.