I did a small photo series on how I inlet rifle barrels. Click this link
[url] http://photobucket.com/albums/v326/Fatdutchman/Flintlocks2[/url]
The photos are in reverse order.
First, I set down the breech into the wood, because this area is sloped down where you cut off the top slab of wood, which establishes the top of the barrel channel of the stock.
Second, I cut the bottom flat to depth. Once this is established, I won't have to mess with the depth any more, other than a little final fitting. I use a plane I made myself to cut a little rabbet just to get me started. Then, I lay the barrel on top of the stock in position, and take diameter measurements at intervals along the barrel (every two or three inches). Divide this measurement in two, and you have the desired depth of the barrel channel. I mark this on top of the stock at the interval marks and chisel down the bottom flat until the depth is achieved all the way down the channel. The depth is done. This is your "stop" for inletting. The bottom flat won't have to be messed with anymore as to depth, though it will have to be smoothed up and widened towards the breech and muzzle ends (with a swamped barrel).
Now, lay the barrel on again, and position it properly centered over the groove, and scribe along each side of the barrel. I then take a wide chisel and chisel straight down just a few taps, going all along both scribe lines, then back-cut to the chisel stab. Then, I use a chisel to cut down these "shoulders" to the depth that I think the side flats will be. It's just by eye. This establishes a "T" shaped inlet.
All that is left is to cut the oblique flats, which is easy to do. THEN you can put your preferred marking medium on the barrel and do your final fitting.
I can usually do a barrel in about 3 1/2 hours (round barrels take longer for me). In fact, I am just in for supper break from inletting one right now. This one is in walnut, which cuts much easier, and I could well get it done in about 3 hours or less...