As I said, pretty any 'blacking/browning' solution will do the trick. Apparently you can even use good ol' sodium chloride solution but I have never tried it.
Most of these solutions are real 'bucket chemistry' and providing you don't try and push the speed of reaction by heating or steaming, keep a close eye on the the first few passes to check for pitting (too violent a reaction) and are patient, a pretty good finish is easy to achieve.
First few passes can look very blotchy and as long as the barrels have been thoroughly degreased (and kept that way!) this will even out by about half way through and be invisible by the end.
Biggest problem is bleeding ribs. Even a tiny pin hole can ruin a length of the barrel blacking beside it and the only real solution is to get the ribs relaid. There are a few bodges (loctite run into the hole comes to mind) but I have never had great success with them.
