I used Bullseye when I formed my first 400 Whelen cases. It was about the only way I had to make cases at the time as I didn't have reloading dies or an expander dies set for it. I used about 16 grains of Bullseye and topped it off with COW and jammed a crayon in the case mouth. It worked well.
However, for the 257 AI I wouldn't bother. Just load up your standard cases and fire away. Unless you just don't want to burn up that many bullets. In that case I think the 4227 would work. I'd start around 12 grains and go up till you come out with well formed cases.
