I took the Parks Canada historic weapons course in the spring. First off - is the cannon and carriage safe to fire? Seriously. Second, do you have the right tools for loading, swabbing, pricking and priming, and for misfires?
You want to fire ashes. They will form a cloud of dust, so be upwind! A .50cal bore is very small considering the volume of ashes to be fired. As the others have offered, blackpowder comes in grain sizes. The smaller the particles, the more power in the firing.
On course we loaded blank charges for 2-inch cannons, so my advice is going to be best estimate. With a dowel smaller than the diameter of the bore, create an aluminum foil sleeve, and crimp over one end. Pour the charge in the open end and crimp over a round cardboard wad. Load the cannon without getting your body in front of the bore. In one single thrust, seat the charge. Here is where my course work will differ from the matters at hand. Put an inch of crumpled newspaper over the powder. Wrap Old Bill like the powder, except maybe in more newspaper. Ram against the charge. Ours had fuse and quick match igniters, so prick a hole in the charge through the touchhole. You should feel the resistance on the prick. Seat the fuse. With a smouldering cotton match, not a Bic or BBQ lighter, start the fuse. Step back 30-40-50 feet on an angle and wait for the earthshattering kaboom. I would expect there to be an equal and opposite reaction of recoil and flash from the mouth.
Remember the internet is free and so is all advice you receive. I was firing inspected cannons under close supervision; you have neither benefit. Nor were we allowed to fire any projectile, only blanks.
