We have had plenty of good answers here. I'd use phosphoric acid, which more or less is naval jelly, after careful degreasing and cleaning. It is dirt that may produce a piebald effect. With the strong acids (and hydrochloric is less dangerous, concentration being equal, than nitric etc.), I would start with them so diluted as to do nothing visible, and increase by stages until it does.
I have seen it said that hydrochloric acid leaves steel very prone to after-rusting. But I have found it not to, if the parts are boiled in a couple of changes of clean water, then oiled. Phosphoric leaves a very fine pearly texture, which blues or case-hardens very well, and looks right on an old gun. If you want bright steel, see what steel wool will do - maybe all you need - or some kind of fine abrasive paste on an old toothbrush or pencil eraser. You can buy your choice of abrasive powders for gemstone polishing from a lapidary dealer, or syringes of diamond paste on eBay at a remarkably low price for diamonds. It may be cubic zirconia, but it works.
