Jon,
Glad to see you are going about becoming involved in BP mortar and cannon shooting the right way. Welcome
As intoodeep said, the easiest way to get into this sport is to buy something ready made. The tube that he mentioned is just fine but
be careful since there's other cannons and mortars out there that can be dangerous. If you find something you like, check with us here about it BEFORE you buy it.
If, instead, you want to be involved in the construction process, check your phone book for steel suppliers and machine shops in your area. A 4" x 7" piece of steel rod for a golf ball mortar shouldn't cost over $50. Like CW said, talk to some machine shops and I'll bet you'll find somebody who will bore it for a very reasonable cost.
If you have trouble finding steel, try:
http://www.metalexpress.netThey list 4"x7" piece of 1018 for $47 while the same size piece of heat treated 4140 (much stronger) costs $48. They have several plants in the midwest so shipping shouldn't be too much.
If you know how to do any sort of wood working, building the base really isn't a problem and you should have some fun doing it.
WARNING...WARNING--- this sport is addictive! After you shoot your first mortar or cannon, you'll be HOOKED. They're like eating potato chips, you can't stop with just one! :wink: