It was 1 3/4". Shot 1 pound patched lead round balls with fair accuracy. It was also perfect for patched golf balls. In theory, it could shoot golf balls over two miles. But unless you had that patch perfectly centered around the golf ball, it would spin off in any direction. Not unlike my golf swing.
Last year I installed a rifled liner we made from a rather expensive SS tube. Now the bore is 1 5/8" (1 in 48" twist) so I am casting the hollow bottom slugs. Using different spacers on my mold (I will post some pictures when I can remember how) I poured 4 slugs yesterday. They weighed in at 17 3/4, 15 1/2, 11, and 8 1/2 ounces. Will get some more poured soon and will try them out in a week or so when I can get back up to the ranch. The ranch is fun, love to make smoke and noise with the cannon, AR's and of course tannerite. But it is all mountain with nowhere to set up a target more than 75 yards.
Up about 100 miles north of us, way back in the hills alongside a reservoir (with cabins & campsites) is a remote dirt airstrip with a sanctioned gun range at one end of the runway. The runway is rarely used and then only by pot farmers who grow illegally in the national forest. It is one mile, clear and flat from one end to the other so it will make a perfect cannon range. Right now it and all roads leading up there are closed due to the current SIP rules. When it opens back up we are going to head up for a day and do some serious testing of powder loads and projectiles to find the optimum combination for distance & accuracy. Maybe even use the chronograph to see how fast those slugs really are. (last time we tried it was too close and blew the chrono down range)