Force a steel round to open as much as possible and then firmly clamp the 'round' in your bench vice and pound the thing back as much as possible.
Next use a larger round, closest to the I.D. as possible, vice again, and back to tap tap 'til its back to original round.
Takes a bit of patience but it can be done.
Another option is to take it to a muffler shop as they have pipe stretchers as well as shrinkers. (they always shrink or stretch same size pipes to make slip joints).