If you are patient in a couple of weeks I will putting up a whole new thread on building a bowling ball mortar including plans. You may be able to shoot a bowling ball with 300 grains, but not far. Bowling ball mortars are big guns and area lot of work and expensive to feed. If you start with a bowling ball mortar you can quickly get discouraged-it is a lot of work.
To start, a pop can mortar is a pretty good size to begin with. Our sponsors can help you there.
As the result of a contest we had here 8-9 years ago I got hooked in on a design I call K.I.S.S. design. I made a number of Golf ball mortars in the design and last year I built a pop can mortar in the design. The bowling ball mortar I am working will be of this design also.
One solid piece oaf advice I can give you before you build or buy, do your research. Find out how a cannon is safely made and how to shoot it. Before you buy a cannon run it by use here. We can tell you the pros and cons. We aren't bashful, we will tell you the truth. Be fore warned, there are folks out there making cannons who have no idea what they are doing. If you question them about and issue, such as a seamless barrel liner, and they respond something like, you don't need that or I have sold a lot of these withn o problem--run!
If you will look at the stickies at the top of the board you will find one called References: Cannon FAQ's, Posting Picture Books, the web, and Cannon Plans. Take a look at it. Lots of good ideas in there. Also read Safe loads and construction.
By the way phsarge, Vrod and Kaboom, welcome to the board!!!