Brass Features When Peterson designed these .338 Norma Mag casings it chose to use the same parent casing as its .338 Lapua Magnum casing. As a result Peterson chose to keep the major diameter of the casing (the diameter just above the extraction groove) the same size as its .338 Lapua Magnum casings. That diameter is .580". Since Peterson has never had any complaints about excessive major diameter growth or primer pocket loosening in its .338 Lapua Magnum casings they believed that to be a fine decision. Brass Specifications Brand: Peterson Cartridge Caliber: .338 Norma Mag Type: Unprimed Brass Count: 50 Diameter: .580