This Link will take to you to
Blackpowder Mortar and Cannon shooting FAQ's Look here for links to posts that answer a lot of your basic questions about shooting Cannons.
Everyone wants to see your cannon. Use this guide on how to post pictures on this board to get started.
BPM&C Photo album links and Picture posting instructions This post also has link to the BPM&C photo albums so you can see some of the cannons we shoot.
The last link is to our Cannon Safety or should we say our Cannon accident post. We can all learn from these post.
Cannon Accidents!!! Safety Warning!!!Warning and disclaimer: Shooting cannons is not like shooting a big over sized muzzleloader. There is a completely different set of internal ballistics involved. Read up on cannon shooting before you acquire a cannon. Suggested reading is
THE MORE COMPLETE CANNONEER By M.C. Switlik with selected excerpts from other artillery manuals The book can be ordered from these two suppliers.
The Complete Cannoneer from Matt Switlikand
South Bend Replica National Safety Rules and Procedures
for Shooting Muzzleloading Artillery, as adapted by the American Artillery Association March 20001. Powder charges should not exceed 2 oz. of Fg or 3 oz. FFA or Cannon Grade Goex powder per inch of bore diameter. No excessive charges. Use black powder only.
2. Prepare powder charges in advance using heavy duty aluminum foil. Baggies may be used inside the foil, taking care not to allow excess air in the baggies and removing excess plastic where unnecessary.
N-SSA National Rules for Cannons10.8.1 CHARGES
Maximum powder charges for all cannon shall be limited to that amount permitted by the chart as published below in Table 10.1. Only commercially manufactured black powder of American standard Fg granulation (150,000 granules per pound/ or 220 granules per 10 gr. wt. (avdp.) sample), or a coarser granulation, may be used. Charges must be wrapped in a powder “bag” fabricated of at least one complete layer of heavy-duty aluminum foil.
For specific load for you bore refer to
N-SSA National Rules for CannonsTable 10.1. Page 71 of 194 in the PDF file.
Moderator note: These rules applies only to guns over 2 inch bore. For guns smaller than 2 inch
THE MORE COMPLETE CANNONEER compiled Agreeably to the Regulations of the War Department as published in "Artillery Drill" by George Patten, 1861 and Containing Other Observations on Antique Cannon By M.C. Switlik with selected excerts from other artillery manuals Appendix I has a load chart which is posted here by permission of the author.

This load chart is maximum load of a unpatched ball attached to a sabot.
Addendum For mortars and howitzers the the diameter of the powder chamber is the considered the bore diameter.
Click on this link to be taken to the post on loading cannon cartridges
Making Cannon Cartridges