Chamber machining of chromium plated M14 barrels is allowed to obtain satisfactory headspace. I'm not saying this is the first choice to get the proper headspace but it is permissible by M14 Military Specification.
Per Military Specification MIL-R-45012E Rifle, 7.62MM: M14
3.3.1.1 Barrel. The barrel shall be free of cracks and seams.
The chromium plating shall be free of nodules, flaking, pits, stripping,
anode burns and evidence of etched base steel, There shall be no
machining, such as honing or grinding, after application of the chromium
plating except as maybe required for obtaining the specified headspace
and dimensional requirements at the muzzle. Burrs and sharp edges shall
be removed from chamber edges. Scratches or marks, occurring in a
chamber which otherwise meets the surface roughness requirements, shall
be permitted provided they do not cause marks on the case of a high-
pressure test cartridge fired in the chamber.