Really really dont want to put a damper on anybody that wants to make a cannon but there are both legal and safety pitfalls (especially with a cast cannon).
The legal pitfalls to cannoneering arose on May 1st 2020 when the govt announced an "order In Council" that prohibited a lot of toys, anything with a bore of larger than 20 mmm( .787" )or can generate more than 10,000 jules of force at the muzzle. Because of the ambiguity of the firearms branch of govt, we cannot get a solid written position on cannon ownership from them. the way the "OIC" is written, it might interest every person on this forum, even the ones that have no interest in the sport shooting aspect of the forum, that for every one of you that even drill a hole of .787" or larger you have created a prohibited weapon and could be subject to criminal charges....a Ludacris statement you may say...then read the OIC and tell me where there is a dispensation for machinist to drill common holes....there quite simply is none.
The second pitfall is the 10,000 jules of force part of the document. I can create that much force with a .30 cal bored unit...and here is the pitfall that any of us haveing a piece of 3/4 inch pipe beside our shop....the document clearly states "capable of producing" ( and that OIC is a legal document presentable in court, not legislated as other laws are, just signed into being), that word "capable" makes every piece of pipe we own makes us subject to the prosecution of any willing prosecutor.
The old cannon controls were a grey area in firearms law and it was more or less regulated that if a tube was fuse or loose powder fired , it fell under an antique category, if primer fired it was considered same as any firearm.
I have several cannons from desktop size mortars& field pieces to very full sized civil war replicas in my yard and they all, every one of them can be fuse fired....and considering all the above...would I build another, you bet!!
The other consideration to consider very seriously on any "cast" tube that you plan on using in a cannon fashion is that casting has long been considered as the least viable way to make a cannon because of the undetectable voids that a casting can have...even BP pressures ( easily attainable to 28,000 psi) can be enough to turn a tube with an occlusion into a grenade .
Modern heavy-wall seamless /DOM steel tubing much better.