I have a 257AI on a Mauser 98 action and a lot of data for it.
It generally runs 100fps more than the standard 257 in a 24" barrel.
I start with max loads for the standard 257 and work up from there.
If you're not comfortable with starting at max 257 loads, although I have never run into problems doing this, you can reduce those loads by 2 grains and start there.
I ALWAYS use standard primers in this cartridge, I have had problems using magnum primers with this round.
If you would like loading data, PM me the bullet weight,brass and powder you would like to use.
I found H4350 to be one of the best powders with 100gr bullets in this cartridge, RE19 and RE22 work well with heavier bullets (110gr,115gr and 120gr.)