Here's a picture of the breech end:
The tube is 1026 steel with 1 inch thick walls, and I'd guess weighs at least 80 pounds complete. I bored it out (from 3 inches) to 3.25 inches for 1 inch deep at the breech end. The plug is a 2 inch long chunk of 3 inch diameter 1018 cold-rolled that I turned something like the shape of a semi-wadcutter revolver bullet: full diameter for 1 inch, then stepped down to 2.75 inches and tapering down to about 2 inches or so diameter at the end. I pounded the plug in flush with the "pointed" end out; that gave a space a minimum .25 inch wide to weld into all around the plug. I had a good welder go in there with a heavy wire welder that used both shielding gas and flux-cored wire, using plenty of amperage and chipping the slag out after each pass. The whole end was changing colors and would have been red hot by the time he finished except he let it cool some.
Then I faced the end clean and made the trunnion piece out of 2.5 inch diameter 1018. It's turned to 2 inch diameter at each end, and I milled a flat about 1.5 inches wide where it fits against the barrel end. I had that welded on, along with the bracket near the muzzle. So the breech is welded 1 inch deep plus both plug and tube are welded to the trunnion bar.
The base is pretty poorly designed and cracked when I fired what I consider to be a safe maximum load -- 2.5 ounces of Fg blackpowder. Such a load keeps that heavy can up there for about 18 seconds, and it really whizzes as it comes back down. I'm not sure of the range as they shoot out of my field and I never could find where they hit out in the woods. I'd guess 300 or 400 yards at that high angle. What surprizes me is how .25 ounce will send the can hundreds of feet up and about 130 to 145 yards horizontally. People put more powder than that into .50 in-lines nowadays. Those loads sink the cans about flush into the ground, but they are not whistling noticeably as they come down.