Back boring is a technique of slightly enlarging the interior dimension of the bore of a shotgun beyond the normal dimensions.
In the 12 gauge shotgun, the nominal SAAMI standard bore diameter is 0.729".
All SKB Competition Series 12 gauge over/under shotguns were produced with oversized bores. These barrels had a nominal bore diameter of 0.735".
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.