The first Mortar I built or rather had built in the 1980's was from a piece of 6 inch monel. I got the plans from a 1970's copy of Guns magazine. When the plans were made beer came in a steel can that took a church key to open. Those cans were bigger around than the now ever popular pop-top extruded aluminum cans. I found out by trial and error that Campbell soup cans fit that gun just perfect.
We filled those cans full of cement and fired them. We were able to put 10 of those cans into 20 feet at 900 yards. We would just point the Tube in a safe direction and fire away.
While I was in gunsmithing school I dimensioned out a hollow based air-gum pellet to the diameter of that gun's bore and made mould for it. I cast two slugs out pure wheel weight that weighed about 8 lbs each.
The Muzzle loading artilleryman had a formula for determining powder charges for cannons by bore size and I made two charges. Turns out that formula was for straight bored guns and not guns with powder chamber smaller than bore size.
I loaded the gun up and fired it off. A huge smoke ring came out about 30 feet in diameter and the slug cracked through the air and disappeared never to be seen again. The recoil broke the 2x6 oak bases in half on both sides. Mind you now the trunnions were bolted into blocks mounted on the 2 inch side of the 2x6 oak sides. The recoil broke those sides like match sticks.
I had this gun along on the trip also.
That's Montana's Sweetgrass Hills in the back ground.
The fella that owned the ranch we were visiting had just pulled a old abandoned Ford Falcon out of the brush. He said why don't you shoot this old car with that cannon. I set the gun up about 20 yards away and aimed at the door. I lit the fuse and got out of the way. Big boom and a cloud smoke. The smoke drifted away and there was a big hole in the car door. We all hooted and hollared a second, when a 6 inch alder about 50 yards beyond the car fell over. Cool we killed that Falcon and a tree too!
I have built two of the navel guns and two parrot rifles since. Well actually I have completed one Parrot rifle and have the carriage hardware to finish on the second. I have built two other Mortars and am working on my fourth. It is the proper bore size to slip fit a diet coke can.
Alas, I am no longer in Montana, stuck here next to VA's beltway. Don't know what I am going to do about shooting around here, but I am already hearing rumors about cannon shooting going on. People jammed so close around her that you can't pass gas least someone complains, let alone do any shooting