Well, they can 3D print metal objects now - I wonder how hard a printed barrel with a bore extractor, muzzle brake and rifling would be?

Now... printing one at a suitable high resolution from one of the resins being used in 3d printers... THAT would get you a nice barrel with detailing, and that would likely be a cost-effective alternative. I can see a lot of potential there - you could have a great barrel with proper taper and configuration, much better details (than that seen on even great slide-molded styrene barrels - I'm thinking here of stuff like those thermal jackets and associated clamps seen on modern tank guns, for instance), and well-formed complex muzzle-brakes all in a ready-to-use one piece barrel - and you wouldn't need to use any of the complex AM PE stuff to build those clamps and jacket details, either!

All this would take RIGHT NOW would be for someone to create the driver file for the printer...!

Even the smoothest printing from Shapeways has what they call "slight grainy feel" - more like a sandpaper surface. They will never be as smooth as a turned barrel. You can sand but at the risk of eliminating some surface detail. Model Monkey and Reedoke use a different printing technique and are almost as smooth as a cast resin piece.