I would think you could take the 3D version and make a sturdy latex and silicone mold of it and then pour in casting wax, then take the now hard wax piece and place it in a box of casting sand. There is a procedure or two involved but essentially you then pour molten metal into the mold which burns off the wax leaving you with a metal piece now, after that its just a matter of fitting the machined sections, a dremel can do most of it.
Usually the lost wax casting process involves pot metal of a soft type of metal. But there are other directions such as now making basically something out of carbon fiber, or an epoxy mix such as JB Steelweld. A lower receiver could work if it had steel inserts in key places, much like a Glock frame.
The TOUGH part is making the barrels. . . but I suspect a carbon-fiber barrel with a wire winding down the lenghth might do for barrels. . .
My old man was a supervisor in a Ford casting plant, know what you’re talking about
And my first thought was ‘some hybrid process’ will make this work... what you say makes a lot of sense, EoU