.257 Roberts
| .257 Roberts | ||||||||||||||||
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| Type | Rifle | |||||||||||||||
| Place of origin | United States | |||||||||||||||
| Production history | ||||||||||||||||
| Designer | Ned Roberts | |||||||||||||||
| Designed | 1920s | |||||||||||||||
| Manufacturer | Remington Arms | |||||||||||||||
| Produced | 1934–present | |||||||||||||||
| Variants | .257 Roberts +P .257 Roberts Ackley Improved | |||||||||||||||
| Specifications | ||||||||||||||||
| Parent case | 7×57mm Mauser | |||||||||||||||
| Case type | Rimless, bottleneck | |||||||||||||||
| 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) | |||||||||||||||
| Rifling twist | 1-10" | |||||||||||||||
| Primer type | Large rifle | |||||||||||||||
| Maximum pressure (SAAMI) | 54,000 psi (370 MPa) | |||||||||||||||
| Ballistic performance | ||||||||||||||||
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| 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 in) and 6 mm (.24 in), and has more energy, but is harder recoiling, similar to larger hunting calibers, such as the 7 mm (.28 in) and 7.62 mm (.30 in).
Nominal bullet diameter of the .257 Roberts is .257 inches. The .257 Roberts uses the same caliber bullets as .250 Savage or the more powerful .25-06 Remington. Barrel rifling diameter, from which calibers derive their designations, is 0.250 in or 6.35 mm.