The phosphate ("parkerizing") used on military arms IS tough. It has to be to resist abrasion and wear. Phosphate coatings work by modifying the surface of the steel. Either iron, zinc, or manganese phosphate is typically used on military small arms finishes. The raw steel is typically sandblasted, and then boiled in the phosphate solution, rinsed, and dipped into preservative oil. Manganese phosphate is the current MIL-SPEC finish, and has been for over 40 years. Hot tank bluing WILL work on a phosphate finish.
To remove your old phosphate finish, you need to either mechanically abrade ("sand"), sandblast, or beadblast the metal.
Why remove it at all? The British used a black ordnance enamel over phosphate for extreme abrasion and rust resistance. European arms did and still do use phosphate finishes and sprayed on epoxy paint. AKMS, AK-74, FN FAL, and HK G-3 battle rifles are examples. Many American gun owners think painted on finishes look "cheap", but they are amazingly durable and practical for military weapons.
If I were refinishing an enameled rifle, I would boil the parts in lye to remove the finish and degrease the metal, rinse, dry, and immediately spray Gunkote on it and bake in the oven. Such a finish would look great, and have almost the durability of a factory paint job.
HTH
John