I've had great results with Brownell's parkerizing solution and Norell's black semi-gloss moly coat. The moly coat is really thin, but very tough and seals the parkerizing. To get the best(even) results you need to use a blast cabinet for 80-90 grit media and a decent airbrush for the moly coat. The parkerizing and moly coat do not change dimensions appreciably. Disassemble, de-grease, blast, inspect, finish blast, de-grease, parkerize, rinse, dry, inspect, moly coat (covers minor park imperfections), inspect, touch up or strip and rework uncured moly coat until satisfied, bake to cure, re-assemble.
It's not economical for one gun. On the other hand, once you work out the process and technique, the cost per gun goes down and it can get addictive looking for cosmetically challenged guns to refinish.