I have done a couple with old style, so I could say to myself "yes, I can do this". Now I true up top and one side, plow a groove almost the length of the barrel, on a table saw, with a dado. Mount the blank in a vise, square out the breech end with chisels, lay the barrel in the groove, use strips of 1/4" thick underlayment (no voids) on each side and breech of the barrel, fairly snug, and tack in place with brads. the router bit is straight sided coming to a point at 45deg., with a bering, the diameter of the bit, that will ride on the underlayment. Works the same with a straight taper, with a lightly swamped barrel the underlayment can be formed to the swamping. If it cant, I lube up the barrel with grease, lay my strips along the barrel touching at the breech and muzzle and fill the gap with "Bondo" or similer product, just before it sets, I pull the barrel. let the filler fully set or harden, the bering will ride on the hardened Bondo. You still have to use chisels and scrapers to finish fit the barrel but a lot of wood gets hogged out in a very controlled manner.