If you wait until September 1st you are going see a bunch of golf ball mortar plans posted here on the board.
That's when all the entrants for the golf ball mortar contest are to post their entry.
The rules of the contest require posting of a drawing.
Your drawing must provide all dimensions suitable for others to make their own copy of your mortar. The drawing can be on the side of an old cardboard box and done with a crayon as long as some one could use the drawing to make their own gun. The mortar must be golf ball bore.