Express Bluing
Express bluing solution may be purchased from Brownell's and that's what the school had for us as an alternative to browning or hot salt bluing (we can also parkerize, color case harden, nickel or cerrosafe at Trinidad State). Instructions are included and they are quite simple and unlike hot salt bluing, can be done at home. It is more durable too than hot salt bluing.
1) Clean the barrel. Acetone to remove the oils but so will other things. I put a dowel rod into the barrel and then put the rod into a vise.
2) heat the barrel. A propane torch passed over the workpiece works. You want it hot enough so that when the bluing is applied, it will sizzle.
3) Apply express blue with a cotton swab.
4) Dump into tank of boiling water for ten minutes.
5) Card.
Repeat 1-5 until desired blue is reached.
The Trapper has been expressed blued:
http://thefiringline.com/forums/showthread.php?t=546403