During the past century, thousands of "decorator" cannons have been manufactured, which were never intended for firing even with blanks. Often the makers would protect themselves by not drilling a vent, or putting in a partial vent that did not connect to the bore. Over time people have drilled the vents through or modified them in various ways so they could fire them. One of the first things we'd probably want to determine is whether the item you have was originally intended for firing safely or not.
Another category of cannon is the salute cannon, of which tens if not hundreds of thousands have been made. These were intended for noisemaking only, not for firing a projectile, and we'd want to see if what you have might be one of those.
