May not work in your case because of the constriction, but a .375NE Wilkes double in Darwin shot one barrel above the other by several inches, and the diagnosis was that it had probably been dropped. The Muzzles were given a gentle twist with a padded Stilson, and it now regulates perfectly. This was done knowing that if the tinning cracked, they were going to need re-regulating anyhow, so nothing to lose really. That rifle still shoots perfectly ~7 years later....

Bush gunsmiths, gotta love 'em.....