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The .257 Roberts Ackley Improved is a wildcat cartridge created by P.O. Ackley by "fire-forming" the .257 Roberts case: the body taper is reduced and the shoulder sharpened to a 40-degree angle, increasing powder capacity while letting you fire factory .257 Roberts ammo in the improved chamber to form brass. It fires .257-inch (6.5 mm) bullets, typically 75–120 grains, and gains roughly 100–150 fps over the parent Roberts, putting it close to .25-06 performance from a shorter case. Valued as a flat-shooting, mild-recoiling deer and varmint round, it remains a handloader's cartridge, as factory ammunition and brass are not commercially standardized.
Suitable Powders
Bullet Weight (gr) -> | 0-50 | 50-75 | 75-100 | 100-150 | 150-200 | 200-300 | >300 |
|---|---|---|---|---|---|---|---|
👍 ... suitable powder
👍👍 ... suitable AND popular powder among our reloading community (clasification might evolve as more community members share their loads )
Caliber Specifications
Standard / Datasheet
Primer Size
Large Rifle (LR)
Case Capacity
63 Grains of Water
Bullet / Barrel Diameter
0.256'' | 6.5 mm
Max. Case Length
2.333'' | 59.26 mm
Max. Cartridge Length
2.775'' | 70.49 mm
Max. Standardized Pressure:
48200 psi | 3323 bar
XXL ID
CID_102
.257 Ackley Improved load data is available with the following bullets:
(most popular first)
