Is the new Kynoch brass like the old paradox brass cases? That is paper lined so the case wall thickness is really that of paper? Most of the brass out there is the thin wall type, not really suited for actual 12 gauge bore diameters (.729-.735). The only reason to use brass cases is if your bore is oversized. Many mistake brass cases as period correct. Thin brass is actually only correct for the larger bore diameter for your particular gauge.