Bullet: 80 gr Tipped TSX  (Barnes)

Diameter: 0.257"

 

Powder: IMR 4350 (Improved Military Rifle)

Starting Powder: 47.5 gr

Max Powder: 51.5 gr

 

Primer: FED 210

 

Case: Winchester Western

Trim Length: 2.223"

 

Cartridge Overall Length: 2.820"

 

Starting Velocity: 3,206

Starting Pressure:  

Max Velocity: 3,537

Max Pressure:  

 

*Barnes loads were developed using brass fire-formed to fit the Ackley Improved chamber. *To fire-form brass, use the faster-burning powders and loads from standard .257 Roberts data. *To properly chamber a rifle for .257 Ackley Improved cartridges, a gunsmith must set the original barrel back about 0.004”. The chamber distance from the bolt face to the junction of shoulder and neck is shorter in Ackley Improved rifles than in standard .257 Roberts chambers. This allows standard .257 Roberts ammunition to be safely fired in .257 Roberts Improved chambers. *The .257 Ackley Improved is a wildcat cartridge, so there is no SAAMI spec for maximum overall cartridge length (COAL). We chose COALs that allowed us to maximize case capacity. Check that the COALs listed for this cartridge will allow the round to fit and feed properly in your particular rifle. Shorten if necessary
Barrel: 24" with 1:10" twist.

 

Source: https://www.barnesbullets.com/wp-content/uploads/2021/05/257AckleyImproved.pdf