.257 Weatherby Magnum - Design and Specifications

Design and Specifications

The .257 Weatherby Magnum shares the same cartridge case as the .270 Weatherby Magnum and the 7mm Weatherby Magnum. The .30 Super Belted Rimless H&H served as the direct parent cartridge for the case design. The .257 Weatherby Magnum was one of the first cartridges which used the shortened, blown out and necked down .375 H&H Magnum case and served as the forerunner to the standard length magnum cartridges such as the 7mm Remington Magnum and the .338 Winchester Magnum. There has been some speculation that Roy Weatherby may have used the full length .375 H&H Magnum case if he had slow burning powders available today when the cartridge was designed. The shortening of the case allowed for the more efficient use of the slow powder of the day, IMR 4350; a powder which would not have provided any great advantage for such cartridges as the .257 Weatherby Magnum or the .270 Weatherby Magnum if such cartridges utilized the full length H&H case. Today, IMR 4350 is considered too fast a burning propellant for the cartridge which comes into its own with the slowest burning powders now available.

Both SAAMI and the CIP have published specification for the cartridge. The CIP standards for the cartridge were published in January 1994.

.257 Weatherby Magnum SAAMI compliant schematic. All dimensions in inches .

SAAMI recommends a barrel rifling contour of 6 grooves with a bore Ø of .2505 in (6.36 mm) and a groove Ø of .257 in (6.5 mm) with each groove being .098 in (2.5 mm) wide. The recommended optional twist rate is one revolution in 10 in (250 mm). Both SAAMI and Weatherby recommend a freebore of .378 in (9.6 mm). Early .257 Weatherby rifles has a twist rate of 1 revolution in 12 in (300 mm) but all current rifles are manufactured with a twist rate of 1 revolution in 10 in (250 mm).

The .257 Weatherby Magnum has a case capacity of 84 gr. of water (5.45 cm3). Sources such as Lyman and Weatherby's pressure rating suggest maximum average pressure limit of 66,000 psi (4,600 bar). CIP limits the .257 Weatherby Magnum to a maximum average pressure of 4,400 bar (64,000 psi).

The .257 Weatherby Magnum features the Weatherby double radius shoulder. The shoulder continuously curves and transitions from the body radius to the neck radius at the point of tangency at the shoulder between the two radii. The SAAMI dimensions for the cartridge reflect this implementation of shoulder to neck transition. While the CIP recognizes the correct radii for both the r1 and r2 values, they treat the transition from the body to shoulder and neck to shoulder as filet radii. For this reason the body and neck dimensions are lengthened and a shoulder angle provided. The treatment of the body-shoulder-neck transition zone accounts for the inconsistencies between SAAMI and CIP official dimensions for the cartridge.