Best product I tried is Evapo-rust (but not used on guns)
Evapo-Rust rust remover description page
From the link...pay attention to the part that it also removes bluing, parkerizing, zinc phosphate and browning!!!
How Evapo-rust affects coatings
EVAPO-RUST is highly recommended by the NRA gunsmithing school and is utilized by FBI, CIA, NATO and other law enforcement and forensics agencies. EVAPO-RUST is perfect for removing oxide weapon finishes such as Bluing, Parkerizing, Zinc Phosphate, and Browning.
Anodizing, Cobalt Tungsten Carbide, Powder Coating, Chrome, Nickel, Paint, and most other coatings will not be removed as long as they do not contain oxides. EVAPO-RUST will not harm lead or solder points."
I believe HarborFreight carry it for $19.99 a gallon. You can also find it at Lowe's and Home Depot.
Another known product is Rustoff. It works faster but note that it is an acid and I won't recommend it.