With an angle grinder it oughta go pretty quickly. If it’s an Ed Rayl barrel it should take 20 hours of filing if you want to go that route.
Whenever I want to really change the profile of a piece of stock I use hacksaw and follow with files. Make a plan for the dimensions at every inch. In this case I’d mount 2 coarse blades in the saw. Mark the waist and there take a certain number of hacksaw strokes crossways on top, side, and bottom flats. Say maybe 8 with perfectly repeatable strokes. As you move from the waist toward muzzle, 7 strokes, then 6 and so on. Very tedious work. Leave a quarter inch between saw cuts. Now do the other 4 flats with your saw cuts halfway between the cuts on the other 4 sides because you always want to be cutting the full width of the flat.
4 hours later start filing till the cuts are gone. Others may approach differently but no way I’d just start filing.
Expect irregularities. All barrels on American guns with forged barrels show irregularities.