Another quirk of published 6.5x53R case dimensions that may cause headspace problems: Wolfe’s Illustrated Reference of Cartridge Dimensions and the Ammo Enzyclopedia list the case with a rim thickness of just 1.25 mm = .049”. I measured some prewar military loads of Dutch, Portuguese and Austrian make. All have a rim thickness of 1.65 mm = .065”, same as a .303 Mk VII load. Btw, rim diameter of the 6.5x53R cases is 13.2 mm = .520” instead of the .303's 13.6 mm = .556” .