May I suggest you have a look at
www.NCAGB.com where you will be able to watch and enjoy a range of cannon being shot at Worcester Norton Shooting Club
www.wnsc.co.uk . Members of the NCAGB [National Cannon Asso of GB] also shoot in postal cannnon comp organised by Douglas .
Although the Ardessa cannon shoots fine, easily cleaned etc when using BP, one should consider certain elements for their robustness~~ a clip on the webside shows Ardessas being used in the postal cannon comp.
Regular shooter cover a variety of bore dia[due to UK legislation on energy restrictions~ refer to table in website]they are maily 18mm/20mm/.69/.577/.75 etc. the balls are normally hand cast with pure lead/jeff Tanner moulds.
I hope the above is useful.