I've owned a .257 Ackley Improved for the last 38 years. It is a pre-64 M70 Winchester with a Douglas Premium Grade barrel. I have taken many antelope, whitetail and mule deer with it using 100 gr. Nosler Partitions chronographed at 3200 fps from its 22" barrel. As far as killing power of one versus the other that's determined by good shot placement using a good bullet. I also own a Weatherby Mark V Euromark in .257 Weatherby Magnum. For antelope and deer I doubt a better cartridge exists that has the same recoil as the .257 AI.