The .257 Roberts, also known as.257 Bob, is a medium-powered.25 calibercartridge. It has been described as the bestcompromise between the low recoil and flat trajectory of smaller calibers such as the.22 and 6mm, and the strong energy but not the strong recoil of larger popular hunting calibers, such as the 7mm family and the popular.30-06. The bore diameter of the.257 Roberts is.250 inches making it a ".25"/6.35 mm caliber", not to be confused with the more well known 6.5 mm bore caliber which uses 6.7 mm/.264" bullets.
History
Many cartridge designers in the 1920s were creating various.25 caliber cartridges. Because of its size, the 7×57mm Mauser case was a common choice, having near the ideal volume capacity for the "quarter-bore" using powders available at that time. Ned Roberts is usually credited with being the designer for this cartridge idea. Eventually in 1934 Remington Arms chose to introduce their own commercial version of such a cartridge, and although it wasn't the exact dimensions of the wildcat made by Roberts, they called it the.257 Roberts. From its introduction until the appearance of more popular 6 mm cartridges such as.243 Winchester and 6mm Remington, it was a very popular general purpose cartridge. Today, although overshadowed by other cartridges, it lives on with bolt-action rifles being available from some major manufacturers.
Conversion of war-souvenir Japanese Arisaka rifles
Japanese Type 38 Arisaka rifles brought to the United States as wartime souvenirs were sometimes converted by rechambering to utilize more readily available.257 Roberts cartridge cases because commercially produced 6.5×50mm Arisaka cartridges were scarce prior to distribution by Norma Projektilfabrik A/S. The neck of the Roberts case would be slightly enlarged to accept handloaded 6.5 mm bullets. The modified Roberts cases are sometimes known as 6.5×.257 Roberts, although the case headstamp may still indicate.257 Roberts. Neither unmodified.257 Roberts ammunition nor the original 6.5×50mm Arisaka ammunition are suitable for firing in rechambered Arisaka rifles.
Performance
With light bullets the.257 produces little recoil and has a flat trajectory suitable for varmint hunting. With heavier bullets it is capable of taking all but the largest North Americangame animals. The original factory load for this is very similar to the.250-3000 Savage.
Improved cartridges
Remington introduced the commercial version of this popular wildcat as a low-pressure round. At the time there were many older actions available of questionable strength. With a modern action and handloading, this cartridge is capable of markedly improved performance. One of the common improvements is called the .257 Roberts which has a SAAMI maximum pressure limit of 58,000 PSI compared to the 54,000 PSI listed for the standard.257 Roberts. P.O. Ackley said that the.257 Roberts Ackley Improved was probably the most useful all-around cartridge. The Ackley Improved was a typical change of a steeper shoulder coupled with blown-out sides for more of a straight cartridge, providing greater powder capacity.
Comparison
Data for muzzle velocity and muzzle energy is for a 24" barrel, except.250-3000 Savage which is for a 22" barrel and.257 Weatherby Magnum which is for a 26" barrel.
