On the difficulties of getting contrast on various barrels (Parker B grade mentioned), I would think the difficulties may be from the two materials being closer metalurgically than a barrel that easily obtains contrasting finish.

If the two materials in the barrels act closer than easily contrasted barrels, the etch would become more critical; too much etch and all the blueing comes off, too little etch and not enough blueing comes off. It may also be necessary for a "pre-etch" of a strong solution of FeCl3 that Oscar mentioned, to "give it some bite". Also, I would try fullstrength rust solution of a few rusts with long dwells before a FeCl3 diluted solution etch.