I'm inclined to agree with TOF, thinking it is more likely nickel and not chrome. Brownell's has some kind of nickel plating remover, but it sounds pretty nasty, and it's not cheap. On a barrel, you might try removing it mechanically first, since the surfaces are long and somewhat consistently straight or round. I haven't tried it, but I would try drawfiling on an octagon, or if it's round, try using abrasive paper in a shoeshine fashion. I may be all wet, but I'll bet some of these guys who have done it will chime in with some real experience.