Dave,
Barrel channels are pretty much as you’ve read.
I don’t think I’ve ever seen one with more than 50% contact between barrel and wood, probably more like 25%. As Mike points out, back at the breech the fit might be better, but the general channel is usually fairly rough with marks from the gouge still evident. Some have flats to match the barrel, but a lot, and already pointed out, seem to be U shaped with little or no attempt to actually form flats to match the barrel except on the vertical sides.
Again, I don’t think I’ve ever seen one with any sign of stain or other finish in it either.
And this is generally the case under butt plates, under patchbox finales, etc, also.
Time was money, so I guess the old boys believed that if you couldn’t see it, it wasn’t worth the time to do a better job. Actually I enjoy seeing this rough work, as you're seeing the hand of the maker and can get a good idea of how he went about his job.
Now I haven’t seen the best of the best so maybe they were done better,,,,, but I somehow doubt it.
John