I ought to describe the process. Short version, you clean and polish the metal- any finer than a 320-grit is a waste- and then very thoroughly degrease it(wearing gloves or doing something to keep any trace of oils or sweat or whatever off it). Then you swab it with a solution that should encourage rust*, hang it up and wait.
When you have a nice, even coat of light, ‘fluffy’ rust, you then card off the loose stuff** with steel wool(degreased with mineral spirits or something) or a VERY fine wire brush; you want to take the loose stuff off but not touch the effect on the surface of the steel. Make sure it’s even all over, then recoat with the solution and set it back up to rust some more. Repeat as needed.
When you’ve got a surface that’s evenly rusted, no spots or uneven places and it looks about right, there’s actually two things you can do. The first is card off the loose and then oil it heavily to kill the solution and rust, let it sit a while, clean it off and oil again; this is browning, gives a very nice medium-to-dark brown finish, very common on muzzle-loaders. I’ve done this on knives to very good effect. For bluing you do an extra step: boil it. Preferably distilled water and a good rolling boil for about fifteen minutes. Done right, it should turn the surface a dark blue. Pull it out, dry thoroughly and oil heavily. That’s the basic.
Do some digging around and you’ll find a lot of variations on the process, and the solution*** and ways to encourage the process. One of the best ways is a bluing cabinet: basically a box tall enough to hang the longest piece you’ll blue, a vent at top and bottom and a tray of water in the bottom; the idea is to keep the humidity high enough to encourage rusting while keeping the stuff out of the way and clean. I’ve heard of taking an old freezer and stripping all the shelves out for one, often putting a light bulb at the bottom to keep the temperature up(sometimes the water on a shelf with the light below). If you build one, I’d suggest painting or coating the inside with something that’ll keep the moisture from affecting the wood too badly.
I’ve mentioned using this on knives, this is the first time I’ve done it on a gun; friend gunsmith has used this process a bunch of times with very good results. We’ll see how it goes. I've got the receiver, inner receiver and lever hanging now.
*I’ve heard of solutions made by boiling water with a little salt, then letting it cool, mixes of water and a little acid and various other chemicals.
**Brownell’s, for one, sells wire brushes, both hand and rotary for a buffer, with extremely fine wire just for the purpose.
***In very dry areas you just about have to use a cabinet; in a humid area can hang it about anywhere, though not in direct rain or mist or whatever. Years back I knew of a guy who built muzzleloaders and did his browning with beer: he’d start heating the barrel with a torch while having a couple, slowing getting it up to about 250F; by the time he got it there he needed to pee and relieved himself as evenly as possible over the barrel or whatever(trigger guard, butt plate, etc.). Got a flat beautiful brown by the method, but I was informed you did not want to be in odor distance while he was doing it.