Though the fluted chamber of a G3 or HK91 leaves striped carbon markings on the fired brass, it cleans up in a tumbler with a minimum of fuss, and can be reloaded without cleaning if so desired. And non-fluted target barrels for the HK91 and police sniper rifles have been fitted to semiauto rifles with no loss in reliability; the fluted chamber feature is largely to preclude case head seperations during extended full-auto fire.
I've also dealt with other MGs, notably the French AAT52, that also use a fluted chamber and really, really need it, and which mark and ding their brass up much worse. In comparison the HK weapons, including the very successful MP5 submachineguns, are reasonably nondestructive of the brass, though the U.S. military rarely deals with reloading once-fired brass...and steel cases really are the coming thing.