Mike and I agree with those who say not to fire your cannon again as it could kill or maim you or others attending the event. Please don't fill it with concrete until you allow us to make you an offer on it. Here is why we are interested: Medieval lore has it that the ancient artillerists used cats to search for cracks within the bore of a cannon which had seen regular use during a campaign. They would fill the suspect cannon bore with water, wait a while for it to fill any cracks and then pour it out and dry sponge. Next they would introduce a curious, thirsty cat to the muzzle and let it go in. Their logic was this, if the cat came out after just a few moments their cannon was good for more use. If the cat lingered, they figured it had found a crack with water seeping out and was lapping it up, thus "proving" that the gun was dangerously defective. It would only be fired by long fuse with the artillerymen shielded by an earthen traverse until the end of that campaign, after which it would be condemned.
Just think about it, my son's little cat, Luna and your tube could be teaching artillery history at gunshows for many years. If you don't want to sell it, start mixing concrete.
Tracy and Mike