The most common Gun Parkerizing for guns desired is a black Manganese Parkerizing
The very best Gun Parkerizing for your gun available is here (Shootersolutions) ShooterSolutions™' Heavy Duty Industrial Manganese.
Why is it the best? Well, besides Manganese being the modern equivelant of Zinc, Manganese resists chlorine better than Zinc - which is why Manganese Phosphate Solutions have been used by Automobile manufacturer for the undercarriage of the Muscle cars.
THe biggest reason it is the best over all others, is that it is true Parkerizing and it is true Parkerizing on steroids: maximum nickel for an engineered-best compromise between nickel and Manganese. Why? Because Nickel is much stronger than Manganese, as corrossion resistant as possible, makes the process self-stopping for "idiot-proofness" i.e. no dimensional changes (bad)for the novice provided you follow the directions and can learn how, with practice to boil an egg. (It is that simple: dunk and go after preperation.)
Don't be fooled with the "home brew Parkerizing" which is NOT gun Parkerizing, but do it yourself industrial phosphate, but no final chromate rinses, special solid oils, etc. Nor the phosphates that (none of them) say they have any nickel in them. Why? No nickel = No Parkerizing. (Phosphate of manganese or zinc only.) AND Parkerizing with nickel is vastly superior in wear resistance to Parkerizing of Copper or black-oxide, as in Factory make-believe Parkerizing. This is why this Parkerizing is superior to any US Gun Parkerizing for off the shelf AR-15's and SPringfield 1911's. (Sad but true.)
Gun Parkerizing, Motorcycle Parkerizing, Auto Parkerizing = Nickel.