Well, I met fellow just this last week who had a desire to build some bowling ball mortars, He has built a few in the past and happened to have a pretty ideal chunk of material. I offered to saw up the 500 lb chunk of tube and was gifted a 20" chunk of it to craft into my own artillery. This is one of the mortars he has built in the past I have done a bit of looking around and am not sure if I want to put a pivot pin under it or have trunnions. I think the pin under is more typical but I like the look of trunnions. I would like depending on how tight the bore fit you might have some issue with welding on heavy trunnions. Anyway I thought I would start a post and maybe it would motivate me to keep after it. The tube needs bored and its too big for my little lathe so its off to the machine shop and I am not sure how long it will take. It only needs about a 1/8" taken out of the bore to be perfect for a bowling ball. The only real drag is after its built I have no idea where I can take the thing for a little test fire session.