The difference between bluing and browning is in the color of the iron oxide. Red oxide (rust) produces the brown, this brown can be converted into black oxide so we get bluing. Just boil the part to be blued in water for 10 minutes, until the red oxide converts into black oxide. I have used rain water, distilled water and reverse osmosis water, all work equally well. Whatever water you use it needs to be soft water so that there is no mineral build up on the parts to be blued.Polishing, 320 grit is lots good for the final polish. The final polish should be lengthwise on the barrel and is best done by hand. I use wet and dry (silicone carbide) paper. The big problem when using abrasive paper/cloth is that the abrasive surface clogs up. This can be overcome by using a liberal quantity of 10 weight oil (hydraulic oil)
It is imperative that the parts to be treated are oil free at all times. Once the parts are degreased never touch them without cotton or nitrile gloves until the final rusting is finished. If there is heavy oil/grease consider a wash down with gasoline first to remove the bulk of oil or grease. Alcohol works really well for the final degrease and between applications of the rusting solution.The rusting solution. There are several commercial preparations available but for me (in Canada) shipping from the USA is obnoxiously expensive. I cast around on the net looking for formulas and most required nitric acid so that brings me back to shipping again. I eventually found a formula where all the ingredients are available locally.
Mixing, just remember the hydrochloric acid is the last thing to be added. The order of the other ingredients does not matter. I do not think this formula would be suitable for browning but it does work well for bluing.
The down side of this formula is that the hydrochloric acid fumes and will rust the hell out of anything in your shop. The solution to this problem is to kill the hydrochloric acid. In a plastic container (that will not make she who must be obeyed go after your wedding tackle with a rusty carving knife) pour in your water and then hydrochloric acid. Now add a common nails. When nail has been eaten by the acid, add another. Keep adding until the acid stops eating nails. Needless to, this is done outside so the fumes do not rust the contents of your shop. Keep the container covered to reduce evaporation but not sealed, the chemical process produces a lot of gas, just punch a nail hole in the lid. A well washed margarine tub is ideal. This will take about 2 weeks before the nails no longer get eaten by the acid. The other benefit of dissolving the nails is that the acid no longer wants to eat iron and this greatly reduces any chances of pitting during the rusting process.
There will be a bunch of black sediment that needs to be filtered out. Grab a plastic funnel and a coffee filter. Filter into a measuring cup so you can get a measured quantity, then transfer the measured amount into the container the solution will be stored in. I am using a beer bottle that has a flip top. A wine bottle that has one of the plastic corks would be good (screw tops get stuck). Now add the measured ferric chloride and alcohol and you are good to go.Applying the rusting solution. I use a folded wad of paper towel that is dampened with the rusting solution but a foam pint brush would work well. The first coat tends to bead up on the steel but if you scrub back and forth the solution bites in and then the surface dampens. The solution dries very rapidly due to the alcohol evaporating. Just one coat at a time is all that is necessary, two or more coats does not help and is likely to cause pitting that you do not want.
You do not want to get any of the rusting solution in the barrels bore or on the inside of an action. It does not matter how careful you are it will get where you do not want it. Shellac (French polish) the thoroughly decreased bore and the inside of actions. Any shellac that gets on surfaces to be blued needs to be sanded off. Do not use alcohol to remove any excess shellac. This means that any between rustings degreasing cannot be done with alcohol but you can use acetone. At the end of the bluing job the shellac is easily removed with alcohol.
Temperature and humidity help speed up the rusting. I live in a cold climate so I use a sweat box.
The two light bulbs are 60 watts each with a lamp dimmer to control the temperature. The baffles are to stop the light (and heat) shining directly on the parts to be blued because the parts get hot from the radiated heat and do not rust. The small pail is for water and there is a piece of paper towel for the water to wick up to aid evaporation. I takes me two to four hours to get a good rusting.
This is where I made my first big blunder. I was expecting to see nice red rust and let the process go on and on waiting for that red rust that never came. I got a dark brown with hints of purple rust and not velvety. What I ended up doing was pitting. It is better to not let the rusting go on and do 10 rustings rather than 5 that are a bit to long. I strongly urge you to practice on an old scrap barrel or round bar so that you can get the feel of it, no one can really teach you this, a case of experience counts.Boiling
You can either buy a tank from a gunsmiths supply or make one. I made mine from an old truck drive shaft that had a 3" lengthwise slot cut in with a cutting torch. A piece of flat bar was welded on at each end and Bob's your uncle. Its a bit rough looking but its cheap and works.
You will also need a pipe burner to go the length of the tank. I set up my burner to use a propane torch I bought years ago. I am currently eying up the burner controls from my old burnt out barbecue, I think there is enough heat there. I do not know enough about burners to offer any useful advice. Check Google.When you boil you can see the rust turn black, it also gets velvety and easy to card of with 0000 steel wool. For corners that are hard to get into a stainless steel suede leather brush is just the ticket. the brass bristled brushes may work but I have not tried one. After the first rusting, boiling and carding you will find the surface is a patch light gray. With each rusting, boiling and carding cycle the color gets darker and the patchiness goes away. Keep on with the cycles until all the patchiness has gone and the desired shade of slate gray is archived. (This slate gray will get darker when you oil the metal)
The final touch.
Remove the shellac with alcohol and oil the barrel. Being cheap I use WD40 but a moisture displacing oil should be used. Thoroughly clean the bore and oil (with gun oil).The blue color when you rust blue is chemically the same as hot bluing and probably more durable.
This whole process sounds like a lot of work, it is not at all. The most time consuming thing is the boiling and that is a good time to have a coffee or other beverage while the water heats up. Small parts I boil on the kitchen stove and that takes about the same time as boiling an egg..
PSAnother way to turn the red oxide into black oxide is by electrolysis. Google "Electrolytic derusting" for more details. I found that electrolysis tends to give more of a black color rather than the dark slate gray of boiling.