I have three of them, one of which I made myself. There was no problem buying the other two shopping on line.
Take a good look “The Complete Cannoneer” by M.C. Switlik.
Golf ball mortar minimum bore size is 1.723”. Diameter of a common golf ball is 1.68”. The extra is called windage.
The formula is (diameter of the projectile/39) X 40.
The picture is a golf ball mortar made from DOM tubing bored out to proper diameter.
This mortar follows plans from this post at Greybeard Outdoors:
https://www.go2gbo.com/threads/k-i-s...7/#post-378007
Plans by Douglas Dickens.