In the picture, the cylinder is shown just as I got it. At the torch cut the steel is 1/4". Since then, the cylinder has been cut to 18" in length and the valve adapter removed. Where the muzzle is now, the steel is 3/8" and, while I haven't measured it yet, the old Mark I Eyeball says that the top of the bell end is over 1/2" thick.
A 4.5" x 5" piece will be welded on that has a 2" x 2.5" powder chamber drilled into it. That puts a minimum of 1.25" of steel around the powder when it's touched off. As I'm sure you know, the chamber is where most of pressure needs to be contained. From what I've been able to find out, as the gas expands into the main body of the mortar the pressure drops very rapidly.
I've talked at length with Bill Tyrell (of Florida Bowling Ball Mortar fame) and am building mine the same way he did his. The plan is to shoot this thing for spectators and all of the shots need to be under 150 yards so the ball can be seen in flight and landing. That means the powder charges will be relatively light.
