I read about using steel rails and a back-saw years ago in Rifle magazine, in an article by John Bivens, and have used the system many times to cut excellent barrel channels.
I made my saw from a piece of an old carpenter's rip saw, about 4" section of the blade. I sharpened it to a prickly edge, and carefully filed off the set from one side. I silver-soldered a length of rod to the top corner and attached a handle. The saw works on the pull rather than the push, and on the top front corner I set a piece of maple to act as an extra handle to keep the blade running straight with pressure along the inside of the rail. Since the set is already on one side, I have to reverse the stock in the vise to cut from muzzle to breech on one side, and from breech to muzzle on the other. In the dark patina of the steel, I have scratched reference lines to tell me when I am at depth at the breech and muzzle, and in the shallower swamp.
The saw does not cut straight down since the set has been removed from the inside edge, and actually curved slightly toward the middle of the channel. You are only going in about 3/8" at most in the deepest part. Once the cut is to depth, and the gouge has taken away the channel wood up to the saw cut, a 1" chisel cuts straight down along the vertical edges of the rails making a perfect outline of the sides of the barrel. It's easier to do than to describe. I think I did a bit of a tutorial on this subject a few years ago?