The AI stands for Ackley Improved. P.O. Ackley "improved" a lot of cartridges that now bear the AI moniker. Mostly, he blew out the sides of the case to make them more or less parallel, rather than tapered, and increased the shoulder angle. The .250AI is a .250/3000 Savage that's been rechambered for Ackley's improved design. If I understand correctly, you would load .250 Savage brass in the improved chamber and fire form the cases to the AI spec's. You gain a little in case capacity and velocity. Whether or not it's worth it is up to you.