I "ran around with" some CW re-enactors and helped care for a seacoast fort in a former life. I remember the Paulson Brothers coming to visit, they build replica cannon for sale, I remember their catalog very strongly stating that all their guns were bored from solid, and that they considered core casting guns unsafe, iirc, due to the chances of impurities getting trapped in the casting, possibly other reasons I don't recall as well . The argument was persuasive for me at least.
It may have been done that way back in the day, but then, if one blew up and killed the crew, probably no lawsuits. We have on display here in Savannah a "chunk" of the breech of a large iron gun, it supposedly almost hit a young R.E. Lee when the tube exploded, he was working for the USACOE on the construction of Ft. Pulaski at the time, and they were testing cannon.
We also have two smaller bronze field pieces on display, a gift from Gen Washington. One of these was "exploded" back in the 1960's and had to be pieced back together for display, but there was some question as to some idjit possibly using modern powder in the tube.
In trying to remember Paulson Bro's, I ran across the following link that may be of use to the original poster.
http://www.cwartillery.org/aguns.html
So, I have a question, I can understand and have seen illustrations on boring the tube, was some sort of portable rig used to turn the trunnions? My profile pick has me running a portable crankpin lathe on a steam loke driver, it was basically a round ring that cutters mounted into which was advanced over the work as it spun. I'm thinking something like this with careful setup..