Chuck, among the newer technologies are plasma-etching and ion milling. They can remove atomic thicknesses. Some of these are used largely in making wafers but they are being used more and more in precision metal and even glass surfaces.
I have a conventional (non-CNC) small machine shop in my cellar and can work to thousandths readily and tenths with a lot of sweat (I'm an amateur machinist at best). Tenths are a breeze for CNC.
Take a look at the page for the Langthorn Hesketh O/U made in Britain.
http://www.longthorneguns.com/cgi-bin/scribe?showinfo=Try-HeskethIt's a new maker and the guy has an ultraprecision manufacturing plant that makes other stuff that he is using to make these guns. I forgot which Italian maker it is but there's one that also does a completely machine-made gun and it goes for about 125K. Langthorn already has the machinery and the technicians to use them so he doesn't have to amortize their cost over a few guns. The horrific cost of this technology is what is making it difficult to apply to fine guns.
The new technologies can be applied to any metal items that are fabricated. Langthorn makes the barrels and receiver out of a single piece of steel. I would love to see one of these shotguns made. I don't own any O/Us and am very tempted to look seriously at one of these.
In the past machine-made guns were things like autos and pumps where tolerances were looser and military rifles with the same considerations.
We might be entering into a whole new world of fine shotguns that are truly machine-made and are as fine and sweet as the hand-worked classics.
I dunno.