One of the things you need to know about your cannon is if the bore is straight.
There is a real simple test you can do.
Get a dowel as close to bore diameter as you can get and at least twice as long or longer than the bore. Place it down the bore. Get a string and tie the string around the dowel. Make sure the knot is tight against the dowel.
Place a pin in your fuse hole. The pin should be a slip fit , not snug, not loose. Pull the string up over the edge of the muzzle and down to the pin in the fuse hole and tie the string tight around the pin. You want the knots tight to the pin and dowel so the string will center on the pin and dowel when tightened.
Stand your cannon up right. Get a straight edge, like a ruler . Hold it upright by one end at arms length. Stand back a ways from your cannon and align the straight edge with the string and the dowel. You will have to move back far enough so that your straight edge covers the full length of the string and dowel. Misalignment at this angle affects windage down range.
Repeat this test by tieing the string on the trunnion. Misalignment in this direction affects elevation.
If the muzzle is centered and your bore is misaligned, that would mean you chamber is off center. The area around the chamber may be thin on one side and thicker on the other. Remember for safety purposes you want at least one caliber of thickness around the chamber.
