When I visited Les Bauska and also spoke with John Buhmiller in Kalispel in 1975, I spent some time straightening rifle barrels in Bauska's shop - lookingup the tubes out the dirty window - I had fleeting visions this was actually happening in Harry Pope's shop. Those who've studied this type of History will know what I'm talking about.
I've helped Taylor 'regulate' his at the range, shooting, removing the barrel, thumping it against a large Aspen, replacing the barrel, shooting again, removing, thumping, shooting - etc, until it shot perfetly.
Some barrels bend more easily than others due to differening alloys used. I am told the thick 20 bore barrels bend more easily than the thin 20 bore barrels. I am referring to muzzle thicknes. There is a big difference between brands.
If it won't shoot centre, then there is little else to do than to bend it - providing sighting won't remedy the problem.