I would start with +P data from Ramshot, and add powder until your .257 Ackley Improved gets 100 fps more.than their listed loads. That's about what I found was the difference between the standard .257 and .257 AI
That is, if it has a 24" barrel.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
