Marksman.
I have a .257 Ackley Improved and have had great success using Berger's relatively new 115 grain "Match Hunting VLD" bullets. As the name implies (very low drag) they have a high ballistics coefficient (G1=.466 / G7 =.239). They suggest a barrel rate of twist of 1 in 10 but mine is a 1 in 12 and they stabilize just fine. You will have to play with the seating depth in the range of +.010 to -.030. Initially you may not be super impressed with 100 yard groups but they are very impressive at 250 yards plus. Bullet performance on medium sized game such as deer and antelope has been very good. For load data I would contact Berger direct. Good luck.

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Doug