Oscar suggested a 15% ferric chloride solution, which I get mixed up by a local chemist. I don't remember exactly, but I think the Radio Shack stuff is 29%, so you'd want to cut it 1/1 with distilled water.
As far as cliff notes go, I can give an abbreviated version of the process.
[1] Coat the bores with something to protect them from the etchant. I use shellac, currently, but expect there are some more modern paints out there that are easier to remove, after.
[2]Rust, boil, and card, as you normally would for standard rust blueing.[No boil, if browning].
[3]Immerse the barrels in the ferric chloride solution for 5-10 seconds, rinse immediately and thoroughly in cold water.
[4]Wet card using 0000 steel wool.
[5]Repeat the process until the desired results are achieved.
There are finer points to this. Such as barrel and room temps can effect the outcome, shorter rusting times seem to help with contrast, even polishing is more important than with standard blacking, etc., but that's the short of it.
Also, I built a tank out of 3" PVC to submerse the barrels in, but I believe it was Doug Mann who brushes the etchant on instead. I had trouble getting an even finish this way, he has no problems with it. I'm guessing that a lot of the end result depends upon the exact metals used, and I've not experimented with brushing since that first attempt. I guess what I'm saying is that you're dealing, somewhat, with an unknown here, as far as barrel make up. Go slowly, and pay close attention to detail. Your material will dictate, to some extent, how long to rust, exposure to the etchant,etc.