I just finished inletting an oct/ round barrel today. It took approx 6 hours, using a gouge, chisels and scrapers. I use a gauge to determine depth, and inlet from the centre line out. The last cuts are the side lines themselves and a straight edge helps with the even depth of the inlet. A candle for blacking the barrel and determining the high spots is a mainstay during the final stages. I tried the "rail" system, and have even used a router in the past, but since I'm not doing this for a living, I actually enjoy the process.
I did make up a plane blade to help with the initial inlet stage, but a gouge and mallet is just as fast IMO.
* I forgot to mention that I inlet deeper than necessary so that I can trim the edges down later and get rid of any imperfections etc which may be caused by taking the barrel in and out.