For what it's worth here are three images of the 4.55" bore dia. mortar - made of 4140 steel and Armaloy hard chrome plated. Barrel and trunion weigh 103 lbs. Fires 4.5 and 7.5 lb concrete filled pipe to about 350 meters. Takes one or two film canisters of black powder. Note that if anything the wall thickness is a bit heavy. You will also note the radii inside the bore - to eliminate stress points. The powder chamber is just about right - could be larger.

http://image1ex.villagephotos.com/pubimage.asp?id_=1320579" width=318 height=408>

http://image1ex.villagephotos.com/pubimage.asp?id_=1320580" width=377 height=412>

http://image1ex.villagephotos.com/pubimage.asp?id_=1320581" width=322 height=409>
Note: all standard disclaimers apply. YOU are responsible for your own actions. If you build something according to these designs, just remember that you've got to be tough if you're stupid. It's your life.