|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, also known as .257 Bob, is a medium-powered .25 caliber rifle cartridge. It has been described as the best compromise between the low recoil and flat trajectory of smaller calibers such as the 5 mm (.22") and 6 mm (.24"), and the higher energy but harder recoil of larger popular hunting calibers, such as the 7 mm (.28") family and the popular 7.62 mm (.30").
The bullet diameter of the .257 Roberts is .257 inches (6.5 mm), not to be confused with the more well known 6.5 mm caliber (like 6.5×55 mm, 6.5 Grendel or 6.5 Creedmoor) which uses 6.7 mm (.264") bullets, or the .25 ACP pistol caliber which is 6.35 mm (.251"). The .257 Roberts uses the same caliber bullets as .250 Savage or the more powerful .25-06 Remington.
Many cartridge designers in the 1920s were creating various 6.35 mm (.25) caliber cartridges. Due to the availability of inexpensive Mauser actions, the 7×57mm Mauser case was a common choice, having near ideal volume capacity for the "quarter-bore" (0.25 in; 6.35 mm) using powders available at that time. Ned Roberts is usually cred 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 6 mm cartridges such as .243 Winchester and 6mm Remington, it was a very popular general purpose cartridge. Today, although surpassed in popularity by other cartridges, it lives on with bolt-action rifles being available from some major manufacturers.
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.
With light bullets the .257 produces little recoil and has a flat trajectory suitable for varmint hunting. With heavier bullets it produces more recoil, but 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.
One of the common improvements is called the .257 Roberts +P which has a SAAMI maximum pressure limit of 58,000 psi (400 MPa) compared to the 54,000 psi (370 MPa) 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.
Data below shows sample muzzle velocities and muzzle energies for a 61 cm (24 in) test barrel, except .250-3000 Savage which is for a 56 cm (22 in) test barrel and .257 Weatherby Magnum which is for a 66 cm (26 in) test barrel.
|Muzzle energy |
|.250-3000 Savage||6.5 g||100 gr||887 m/s||2,911 ft/s||2,552 J||1,882 ft⋅lbf|
|.257 Roberts ||6.5 g||100 gr||920 m/s||3,020 ft/s||2,746 J||2,025 ft⋅lbf|
|.257 Roberts +P ||6.5 g||100 gr||940 m/s||3,090 ft/s||2,870 J||2,120 ft⋅lbf|
|.257 Roberts Ackley Improved ||6.5 g||100 gr||983 m/s||3,226 ft/s||3,133 J||2,311 ft⋅lbf|
|.25 WSSM||6.5 g||100 gr||1,010 m/s||3,313 ft/s||3,305 J||2,438 ft⋅lbf|
|.25-06 Remington||6.5 g||100 gr||1,013 m/s||3,324 ft/s||3,327 J||2,454 ft⋅lbf|
|.257 Weatherby Magnum||6.5 g||100 gr||1,070 m/s||3,512 ft/s||3,714 J||2,739 ft⋅lbf|