I have used Birchwood Casey Plum Brown. It produces a rust finish. It's not that difficult and if done properly, looks pretty good on older guns.

Basically, you remove the blue finish; smooth out all surfaces with steel wool or a grey scotch brite pad; heat the metal with a torch or heat gun till it's hot to the touch; rub a rag soaked with the solution on the hot metal (use adequate ventilation); neutralize the solution with water; dry and apply a light coat of oil (Remoil or equal); and you're done.

It sounds more complicated than it really is. Having done it to several very old rifles, I found it really easy once I overcame the fear of screwing it up. You can always redo it if it comes out splotchy.

Keep in mind that you will end up with a Model 15 with a rust finish. If you try it and decide to try cold bluing instead, just take a wire wheel to it and finish off with the scotch brite pads.

Hey, I just looked at the DIY article above. What I was referring to, is not that process.