257 Roberts Case Dimensions

The 257 Roberts, also affectionately known as the Bob, is a classic medium-powered quarter-bore centerfire rifle cartridge created by necking down the legendary 7 by 57mm Mauser case to accept 257-thousandths-inch bullets! Developed as a wildcat in the 1920s by Ned Roberts and commercialized by Remington in 1934, it bridges the gap between lightweight 6mm match rounds and heavier hunting calibers.By utilizing an efficient long-neck design and a gentle shoulder angle, the 257 Roberts delivers exceptional accuracy, mild recoil, and superb terminal ballistics for medium game hunting.

257 Roberts Case Dimensions Overview

Dimension PropertySpecification (Imperial)Specification (Metric)
Parent Case7 by 57mm Mauser7 by 57mm Mauser
Bullet Diameter257 thousandths in6 mm and 53 hundredths mm
Case Length2 inches and 233 thousandths56 mm and 77 hundredths
Case Trim Length2 inches and 225 thousandths56 mm and 52 hundredths
Overall Length (COAL)2 inches and 775 thousandths (Max)70 mm and 49 hundredths
Rim Diameter473 thousandths in12 mm and 1 hundredth
Rim Thickness50 thousandths in1 mm and 27 hundredths
Base Diameter472 thousandths in11 mm and 99 hundredths
Shoulder Diameter430 thousandths in10 mm and 92 hundredths
Neck Diameter290 thousandths in7 mm and 37 hundredths
Shoulder Angle20 degrees20 degrees
Case CapacityApprox 51 grains water3 cubic cm and 31 hundredths
Maximum Average Pressure54-thousand psi372 megapascals

Understanding the 257 Roberts Case Dimensions

  • Parentage:Derived directly from the military 7 by 57mm Mauser case, retaining its robust body taper, large rifle primer pockets, and generous powder volume profile;
  • Shoulder Geometry:Features a gentle 20-degree shoulder angle that promotes ultra-smooth feeding from internal magazine boxes and effortless case extraction in traditional bolt-action rifles;
  • Neck Length:Incorporates an exceptionally long neck that provides excellent bullet alignment and deep neck tension, which contributes heavily to the legendary inherent accuracy of the cartridge;
  • Action Compatibility:Spans an intermediate length that traditionally requires a slightly longer action than standard short actions, making it a favorite in long-action or intermediate-action custom builds!
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257 Roberts Case Dimensions in Inches and Millimeters

Measurement PointMax (Inches)Min (Inches)Max (Millimeters)Min (Millimeters)
Case Length2 and 233 thousandths2 and 183 thousandths56 and 77 hundredths55 and 45 hundredths
Trim-To Length2 and 225 thousandthsNone56 and 52 hundredthsNone
Rim Diameter473 thousandths463 thousandths12 and 1 hundredth11 and 76 hundredths
Base Diameter472 thousandths468 thousandths11 and 99 hundredths11 and 89 hundredths
Shoulder Diameter430 thousandths425 thousandths10 and 92 hundredths10 and 79 hundredths
Neck Diameter290 thousandths285 thousandths7 and 37 hundredths7 and 24 hundredths
Bullet Diameter257 thousandths255 thousandths6 and 53 hundredths6 and 48 hundredths
Overall Length (COAL)2 and 775 thousandths2 and 550 thousandths70 and 49 hundredths64 and 77 hundredths

257 Roberts Case Length

The maximum case length of the 257 Roberts is 2 inches and 233 thousandths (56 mm and 77 hundredths), with a recommended trim-to length of 2 inches and 225 thousandths (56 mm and 52 hundredths)!Because of its moderate pressure limits (standard SAAMI max pressure is 54-thousand psi, while plus-p loads push around 58-thousand psi), brass stretching is relatively slow compared to modern high-pressure magnums! However, handloaders should still measure and trim cases after a few firing cycles to maintain consistent neck tension and chamber safety!

257 Roberts Base and Rim Dimensions

The 257 Roberts inherits its robust rim and base architecture straight from the Mauser lineage!

  • Rim Diameter:473 thousandths in (12 mm and 1 hundredth)
  • Base Diameter:472 thousandths in (11 mm and 99 hundredths)
  • Rim Thickness: 50 thousandths in (1 mm and 27 hundredths)
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This classic Mauser rim size ensures absolute reliability in extraction and broad compatibility with standard bolt faces designed for 7 by 57mm, 308 Winchester, and 30-06 Springfield families!

257 Roberts Shoulder and Neck Dimensions

  • Shoulder Angle: 20 degrees provides smooth feeding characteristics and reduces bolt thrust during ignition;
  • Shoulder Diameter: Measures 430 thousandths in at the shoulder junction;
  • Neck Length & Diameter:Features a long neck spanning nearly the full diameter width, with a neck diameter of 290 thousandths in, allowing excellent grip on both light varmint bullets and heavy game projectiles!

257 Roberts Case Capacity

The internal case capacity of the 257 Roberts averages roughly 51 grains of water, varying slightly depending on brass manufacturers like Winchester, Remington, or Peterson! This capacity strikes a sweet spot for medium burn rate powders like Hodgdon H4350, IMR 4350, and Reloader 19, pushing 100-grain to 117-grain bullets at exceptionally efficient velocity nodes without burning out barrels!

257 Roberts Dimensions vs Similar Cartridges

CartridgeBullet DiameterCase LengthCOALMax Pressure
257 Roberts257 thousandths2 and 233 thousandths2 and 775 thousandths54-thousand psi
6-5 by 55mm Swede264 thousandths2 and 165 thousandths3 and 150 thousandths51-thousand psi
243 Winchester243 thousandths2 and 45 thousandths2 and 710 thousandths60-thousand psi
25-06 Remington257 thousandths2 and 630 thousandths3 and 250 thousandths63-thousand psi
7 by 57mm Mauser284 thousandths2 and 235 thousandths3 and 65 thousandths51-thousand psi

Chamber and Case Dimensions

Chamber specifications for the 257 Roberts are tailored around traditional throat lengths to accommodate factory seating depths safely! Many older rifles feature generous freebore, allowing handloaders to seat bullets further out toward the lands if magazine length permits! Always verify your specific rifle throat configuration when working up custom handloads to ensure optimal precision!

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Common Questions

  • Can I form 257 Roberts brass from 7 by 57mm Mauser brass?Yes! Because the 257 Roberts is a direct necked-down derivative of the 7 by 57mm Mauser, running 7 by 57mm brass through a full-length 257 Roberts sizing die is a common and easy method to form custom cases!
  • What is the difference between standard and plus-p 257 Roberts loads?Plus-p loads are loaded to higher chamber pressures (around 58-thousand psi instead of 54-thousand psi) to take advantage of modern strong rifle actions, delivering faster muzzle velocities for hunting!
  • What twist rate is ideal for 257 Roberts dimensions?A traditional 1 in 10 twist rate handles standard weight bullets up to 117 grains perfectly, while modern custom barrels with faster twists can stabilize heavier custom high-BC projectiles!

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