The slow rusting method of browning the metal uses a solution and the humidity in the room to cause a thin coating of hard, brown rust to form on the metal.
Many of us really like Laurel Mountain "Barrel Brown & Degreaser".
Here's a link to it
http://www.trackofthewolf.com/Categories/PartDetail.aspx/388/1/LMF-BROWN
To use it, apply a very light coating on the metal without overlapping the coating.
Place the barrel in a humid area like a steamy bathroom. After an hour or so the rusting will start. It may look streaky after only one coating but don't worry.
After a few hours, take the barrel and rub off any loose rust using a course piece of fabric and apply another coating of the solution, then back to the humid area.
This process takes time and you may have to apply 3-5 coatings to get it to look uniform.
After a uniform coat is made you have several choices available.
You can just oil it to make a very durable browned finish.
You can change the color from brown to black by pouring boiling water onto the brown rust. (You must do this before applying any oil and it will take several dousings with the boiling water).
When it is blackened to your liking, apply a coating of oil to finish it.
You can use a course steel wool to scrub off as much of the browning or blacking as you want to give it an aged look. Then apply the oil.
This scrubbing is not easy. As I said, this is a HARD rust that is very wear resistant.