or are you looking for something that creates an iron phosphate finish? It is more of a grey finish. There is something used before painting steel, I believe it often referred to as 'ospho'. And the end result may be similar to the above "Oxpho" method.
Not sure how durable or useful for gun parts, I've never used it for that.
Others have used some different chemicals to create a matte grey finish. Some of those methods have been frowned upon, but I've never used them either.