Yes, there is a considerable amount of "Air Space" between the bbls & under the ribs on a conventional bbl set. I too worked for 35yrs as a machinist & put in quite a bit of time on CNC Milling Machines. Thet are marvelous machines, but all machines have their limits. Bores would have to be put in on precise angles & even the best of gun drills will veer a little sometimes from a perfectly straight line (I have been exposed to them also). Conventional bbl machining is also much different from what it was in the late 1800's. I don't know how many makers are using it but the "Roto-Forge" method of making bbls Ithaca began using in the early 80's could spit a perfectly straigt, round & concentric bbl out in seconds, starting with a blank tube about 12" long as I recall which wasted no metal at all. Tubes produced by such a method & combined with a CNC machined Mono-Block would be vastly more economical than machine the entire bbl, "+" it allows a different mat'l to be used for the lug area giving a different heat treat from the bbls, another consideration of some importance.
There are simply a lot of factors to be considered, no doubt the companies which build them have considered most of them.
