Brian,
I have been using a homemade Niedner formula from Dunlap's book, but have heard good words about the Laurel Mountain stuff.
Polish,cleaning and sweat time are critical. At least worn 320 polish. Boil with real TSP to clean, boil again to rinse Too long sweat time will etch surface.
4140 takes more coats than mild steel, but works fine 
Protect bore and other surfaces with shellac, several coats. Remove shellac with denatured alcohol and some scrubbing.
Chuck

Some things I've learned from doing this:
The barrel has to be CLEAN of all grease and oils.
Put the solution on sparingly! I've put the solution on with a foam painters brush (works well on flat surfaces)and also made a pad out of a cotton ball with a piece of clean old dish towel wrapped and tied around it.  There is a tendency for a drop to form where you come to the end of the barrel.  Don't leave it. It will form a deeper etch.
Wear new cotton gloves and don't touch metal with bare fingers.
Make sure the barrel has no water droplets on it anywhere before you start carding. Like Deadeye  says, blow the water out of the screw holes with a compressor. Leaving water droplets will cause spots
Do not leave any rust on the barrel after carding and before you start the next coat as this will lead to pitting.
If you don't have a carding wheel and use steel wool, wash it out in acetone and make sure it's dry.

If you don't have several days to do a Slow Rust Blue, you can use the Express Blue Method in a few hours.  Warm the parts gently with a propane torch before applying the solution.  This will cause the parts to rust immediately.  You want the solution to evaporate right away but if you get the parts too hot, the solution will sizzle and this will cause spotting.

Have found grocery store rubber gloves work well when rust bluing. They do come in large sizes. Scrub them regularly on your hands with hot water, Ivory soap, and a scrub brush. No finger prints yet.
Chuck

Use a bent-up brazed stainless tank for boiling. Would not use it for hot blue. Would like comments on Galvanized steel for a boiling tank. Don't think it would hurt.

Tri-sodium phosphate from a paint store. Have seen some labeled TSP that say "Contains no phosphate". Suspicious, have not tried it when I can get the real stuff.
Chuck

I don’t have any firsthand experience with galvanized tanks for rust bluing, but, I’m very cautious about using galvanized metals around any finishing processes.  Zinc is the primary metal in most galvanization, it is designed to be sacrificial, meaning that zinc is a softer material than the host material, so it’s designed to react (corrode) before the base metal does. 

Zinc reacts quickly to bases and acids; hot dip bluing solutions are primarily lye which will quickly dissolve the zinc, the zinc will then leave ugly specks on any parts blued in the solution. Zinc also reacts with acids, most rust bluing solutions are acid based. I have not measured the acidity of the water tank after rust bluing, but I suspect it’s mildly acidic.  If so, the acidic water would quickly dissolve the zinc, and cause the same sorts of problems as it would with lye based solutions. 

Historically I’ve used the black iron tanks sold by Brownells, they would generally last me 2-3 years before they corroded completely thru. The last tank that I bought was one of Brownells stainless tanks, I suspect that it will outlast me. 

For degreasing barrels, I’ve used TSP, and it works well. For the past few years I’ve been using a product called “Greased Lightning” find it in most hardware stores and even Wal Mart. The primary ingredient in GL is Sodium Hydroxide (Lye), that and a little Dawn dish soap makes a darn good degreaser. Pure lye is getting harder to find, many folks will use a drain cleaner which contains lye, but, many drain cleaners also contain aluminum flakes; this produces an exothermic reaction which causes heat, which helps break down grease clogs. Unfortunately the suspended aluminum can also attach itself to the base metal of the barrel causing unsightly silver specks on the finished blue. 

For the OP, if this is a onetime project, it might be better (cheaper/easier/better results) to send it out to someone that does rust bluing. The equipment needed, while not overly expensive can exceed the cost of a good rust blue done by a professional. 

I'd advise you to stay away from anything galvanized.  I once, and only once, used a galvanized nail to insert into a trigger mounting hole to turn the barrel while applying the bluing solution.  I forgot to remove the nail for the next rusting/boiling cycle and ruined the job.  Had to sand it all off and start over.  I use LMF since it's all I can get mailed to Alaska and can't buy the chemicals to make up a solution.  I use a folded piece of cotton shirt to apply.  Soak the cotton the day before, squeezing out any excess, and store it in a baggie.  This distributes the solution evenly.  Too much solution applied will keep you at the job forever.  For the final 2-3 coats your applicator should feel almost dry.  I don't plug the bore and dry it as mentioned above, with a couple of patches and compressed air while holding it with a few folded up paper towels.  I degrease with Zylene, outside 'cause I can't stand the smell.  I use a humidity box and let it rust over night if I want a soft and really durable blue.  There's a learning curved involved but you're getting some good advise here on the board.  Be fearless and forge ahead!

JS

This has been a wonderful post! Lots of great information so far!