Instead of starting with a .729" bore and removing metal from the inside to create the larger diameter, SKB actually manufactured a .735" bore to begin with.

Bore diameterof bearing

Bore diameterof cylinder

Back boring is a technique of slightly enlarging the interior dimension of the bore of a shotgun beyond the normal dimensions.

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All SKB Competition Series 12 gauge over/under shotguns were produced with oversized bores. These barrels had a nominal bore diameter of 0.735".