It is really the fact that the investment cast receivers, cylinders, bolts, etc. are made from chrome-moly alloy steel very similar to 4140. The chrome inhibits rusting enough so that rust bluing is difficult to do.
RAN
I have a Sako L579 with a Douglas barrel (chrome moly 4140, I think) that is ready to be blued (in the white). I want to rust blue it for the classic blued look and durability. Is rust bluing not as advisable for modern alloy steels as some other bluing? I've got the Laurel Mountain Forge browning/blueing solution, a rusting box and steam tube made, but if some other bluing method will better give me the look and durability that rust blue is known for, I'll go with that.
I also wanted to put Gun Kote (the nano ceramic stuff) on it after bluing. Is there something to stop the bluing process that doesn't require oil (because you have to degrease to put the Gun Kote on).. I'd rather put oil over the Gun Kote, if possible; it really helps prevent unwanted rusting of blued steels in my experience.