Wouldn't it be easier to lay out the profile of the swamp for two opposing flats using layout dye and a long straight edge on the oblique flats to either side of the "target" flats? Then cut the waist to depth on the two opposing flats, by sawing almost to depth and filing the ridges between the cuts, then repeating that process up the taper, filing to the lines progressing from the waist to breech and muzzle. Then repeating the process on the two opposing 90 degree flats, and so on.
I have never swamped a barrel by hand, and may never do it, but laying out and filing or grinding to the lines seems easier than making tons of measurements that, IMHO only complicate the process.
As to stress relieving the barrel prior to filing, would heating the entire barrel in a trench of hot coals and allowing it to cool in the coals provide the temps necessary to properly relieve those stresses?
I wonder if hack sawing cuts along the barrel and removing the bulk of metal with a side grinder might be easier...if sufficient care was taken to prevent cutting too deep. The bottoms of the cuts might work as a depth gauge to prevent grinding too deeply. Grinding would have to progress very slowly to prevent heat build up on the flats being ground.
Just kinda thinkin'...tying out loud, so to speak.
J.D.