Parker Ackley's classic improvement of the .257 Roberts — blown-out case with a 40-degree shoulder adds roughly 100 fps over standard .257 Roberts, with better brass life and more consistent ignition. One of Ackley's most successful and widely adopted improved cartridges.
Type: Rifle
Introduced: 1945, United States
Parent case: .257 Roberts
Standardization: Wildcat
Bullet diameter: 0.257" (6.53 mm)
Max pressure: 54,000 PSI
Rim type: Rimless
Primer: Large Rifle
Typical twist rate: 1:10
Muzzle velocity: 3000–3400 fps
Muzzle energy: 1740–3080 ft-lbs
Effective range: 400 yd
Common bullet weights: 87, 100, 115, 120 gr
Primary use: hunting, varmint
Production status: Active
Also known as: .257 AI · 257 Ackley Improved · .257 Roberts Ackley Improved · .257 Roberts AI · .257 Roberts +P AI
History: Parker Ackley developed his "Improved" series of cartridges in the 1940s–60s by reforming commercial cases with a sharper 40-degree shoulder and minimal body taper. The .257 AI quickly became a favorite among deer and varmint hunters wanting more from the .257 Roberts platform without moving up to a belted magnum. Still widely reloaded and chambered by custom rifle makers.