Expansion reamer, either with the driving shank or arbor "piloted" (bushed) in the bore and driven from the breech OR driven by a T-handle from the muzzle end by an operator who attempts to hold his reamer and his tongue just right. Latter method is said to work best with chokes which have a parallel-walled section at the muzzle following the tapered section. Presumably there are dedicated lathes and boring machines which assure the "precision" which sometimes allows POI to be displaced at least as much as would be possible for a blind man with a rattail file.
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