|Place of origin||United States|
|Variants||.257 Roberts (+P), .257 Roberts Ackley Improved|
|Parent case||7×57mm Mauser|
|Case type||rimless bottlenecked|
|Bullet diameter||.257 in (6.5 mm)|
|Neck diameter||.290 in (7.4 mm)|
|Shoulder diameter||.430 in (10.9 mm)|
|Base diameter||.472 in (12.0 mm)|
|Rim diameter||.473 in (12.0 mm)|
|Case length||2.233 in (56.7 mm)|
|Overall length||2.775 in (70.5 mm)|
|Primer type||large rifle|
|Test barrel length: 24
Source(s): Accurate Powders 
The .257 Roberts a medium powered .25 caliber cartridge known affectionately as the Bob. It has been described as the best compromise 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.
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" (called this because the .25 caliber is one quarter of an inch) 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 6.5x50mm 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 6.5x.257 Roberts, although the case headstamp may still indicate .257 Roberts. Neither unmodified .257 Roberts ammunition nor the original 6.5x50mm Arisaka ammunition are suitable for firing in rechambered Arisaka rifles.
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 American game animals. The original factory load for this is very similar to the .250-3000 Savage.
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.
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.
|cartridge||Bullet Weight||Muzzle Velocity (ft/sec)||Muzzle Energy (ft·lbf)|
|.250-3000 Savage ||100||2911*||1882*|
|.257 Roberts ||100||3020||2025|
|.257 Roberts (+P) ||100||3090||2120|
|.257 Roberts Ackley Improved ||100||3226||2311|
|.25 WSSM ||100||3313||2438|
|.25-06 Remington ||100||3324||2454|
|.257 Weatherby Magnum ||100||3512**||2739**|
Using a 24" barrel except:
* using a 22" barrel.
** using a 26" barrel.
- "Accurate Powder Reloading Data Guide, 5th ed.". Western Powders. pp. 29–30. Archived from the original on 2013-07-14. Retrieved 12 July 2013.
- The .257 Roberts (.257 Roberts +P) by Chuck Hawks (membership required)
- Barnes, Frank C. (1997) . McPherson, M.L., ed. Cartridges of the World (8th Edition ed.). DBI Books. pp. 64, 91. ISBN 0-87349-178-5.
- The .257 Roberts by Chuck Hawks
- Speer Wildcat Rifle Loads Speer Products Company (1956) p.35
- Davis, William C., Jr. Handloading National Rifle Association (1981) ISBN=0-935998-34-9 p.170
- Alan Serven. "SAAMI Velocity & Pressure Data: Centerfire Rifle". SAAMI. p. 5. Archived from the original on 2013-07-14. Retrieved 2013-07-12.
- Ackley, P.O. (1962). Handbook for Shooters & Reloaders. vol I. Plaza Publishing. ISBN 978-99929-4-881-1.
- "Nosler Reloading Guide 7, 250-3000 Savage". Nosler. Archived from the original on 2013-07-14. Retrieved 12 July 2013.
- "Nosler Reloading Guide 7, 257 Roberts Ackley Improved". Nosler. Retrieved 12 July 2013.
- 6.5×55 load data from Hodgdon