Rifle · Wildcat
.257 Ackley Improved
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.
- Caliber
- 0.257″ · 6.528 mm
- Introduced
- 1945
- Case
- Rimless
- Parent
- .257 Roberts
- Primary use
- hunting, varmint
- Status
- Active
- Introduced
- 1945
- Type
- Rifle
- Origin
- United States
- Inventor
- P.O. Ackley
- Manufacturer
- P.O. Ackley
- Standard
- Wildcat
- Status
- Active
- Availability
- Specialty
- Case capacity
- 70 gr H₂O
- Twist (typical)
- 1:10
- Primer
- Large Rifle
- Case type
- Rimless
- Max pressure
- 54,000 psi
- Eff. range
- 400 yd
- Recoil
- 9–13 ft·lb
Ballistics
Velocity only, no drag model: no published ballistic coefficient is on file for .257 Ackley Improved, so this is the shape gravity alone gives the advertised velocity. Drag would steepen it. The tool below solves real loads through their published G1 or G7 model.
Open in Ballistic Compare- Parent case
- .257 Roberts
Representative trajectory — modeled from a single velocity input (no published BC for this cartridge yet).
Dimensionscase measurements · scale drawing · size vs familiar rounds
Case dimensions — the scale drawing at the top of the page is built from these.
reloading data · secondary source| Measure | mm | in | ✓ |
|---|---|---|---|
| Bullet ⌀ | 6.53 | 0.257 | · |
| Neck ⌀ | 7.37 | 0.290 | · |
| Shoulder ⌀ | 11.56 | 0.455 | · |
| Shoulder angle | 40° | — | · |
| Base ⌀ | 11.96 | 0.471 | · |
| Rim ⌀ | 12.01 | 0.473 | · |
| Belt ⌀ | n/a | ||
| Rim thickness | 1.25 | 0.049 | · |
| Neck length | 8.31 | 0.327 | ≈ |
| Case length | 56.72 | 2.233 | · |
| Overall length | 70.49 | 2.775 | ~ |
Size vs familiar rounds
Drawn at true 1:1 — every case is accurate relative to the others. Absolute size depends on your screen's pixel density, so measuring one against a real ruler may be off. .257 Ackley Improved is highlighted below.
Reloading costestimate cost per round vs factory
Estimate your cost per round and how it compares to factory. Inputs are yours — nothing is stored.
Cost estimate only — not load data. Charge weight is your input; follow published manuals for safe charges.
Lineage
Connected reference
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.
FAQs
- What twist rate does .257 Ackley Improved use?
- .257 Ackley Improved typically uses a 1:10 twist rate.
- What bullet diameter is .257 Ackley Improved?
- .257 Ackley Improved uses a 0.257″ (6.528 mm) diameter bullet.
- Is .257 Ackley Improved still in production?
- .257 Ackley Improved is active; typical availability is specialty.
- What is .257 Ackley Improved used for?
- .257 Ackley Improved is primarily used for hunting, varmint.
- What is .257 Ackley Improved based on?
- .257 Ackley Improved is derived from the .257 Roberts case.
Sources & provenance
Data & sources. Specs compiled from the Lindcott Armory reference; the trajectory is modeled (point-mass), not measured. See methodology. Spotted an error? Report it →