My experience is with M14 rifle barrels and it probably can be applied to most other chromed barrels.
Chrome plating is a process that adds thickness to a metal part. When the M14 barrel is manufactured, all parts that are chromed -- the chamber and bore -- are made slightly oversize. The chrome plating process is applied to these areas and builds them up to the correct size. Some final lapping may be necessary to bring them into specification, though.
If you were to chrome plate a standard steel barrel, the chamber and bore would have to be made oversize to be brought back to specification by the plating. The time to plate the barrel is at its initial manufacture, not after it is made.