"Like one of the other posters said, you can drop down one gauge if you like when you sleeve the barrels. Toby's men can sleeve a 12 to a 16 gauge usually with no problem."
You can certainly do that; but I contend you won't get the finished look you'll be satisfied with if you're as anal as I am.
All my sleeving/mono-blocking activities took place in the 1970's and 80's; and I remember a couple of old 16-bore Damascus Parker guns with which I was infatuated, so I sent them to Del Grego for restoration. My request, among other requests, was that these guns be mono-blocked back to 16-bore (love the 16-bore); in both instances Mr. Del Grego advised me that he MUST re-tube these barrels to 20-bore, as the "barrel walls would be too thin at the joint and would therefore be unsafe". I was a lot younger and very inexperienced in those days; but I now believe that his claim was pure unadulterated BS. At any rate, I agreed but the end result was an elimination of the nice original factory tapers built into the tubes; and necessary where barrel wall thickness is gradually reduced between the breech and hinge pin area. The mono-block was created by chopping barrel tubes just past the hinge pin; then joining the smaller 20-bore tubes to the larger 1arger diameter 16-bore tubes, which resulted in a noticeable "angle" at the new tube joint. I've also seen a Quality 7 LC Smith that was factory re-barreled from 10 to 12-bore; the resulting "look" was very similar; but then I can accept/excuse such factory work. Back in the stone age before we understood what we now know about Damascus tubes and mono-blocking was common, I also saw my share of 16-bore Parkers that had been mono-blocked to 28-bore; but I never saw an example that "looked right", and every example I examined was on the heavy side. Obviously a man can do with his gun as he pleases, but I would never recommend mono-blocking a quality double to a smaller bore size.
