Thanks for the comments all. Thanks for the link to your investigation pab1. (side note, how do you search for 45-70 using the search function? I tried "45-70" and didn't get anything). I got some info back from Barnes wrt 0.458 cal bullets that I thought I would share. The minimum velocities listed are to start bullet expansion, but for optimal expansion add 100-200 fps. The bullets are tested to the upper limit velocity and must retain 100% weight (excluding plastic tip) at that velocity.

Barnes bullet #30640 and #30642 for the 458 SOCOM are the same bullet with suggested impact velocities of 1100-1900. They suggest using the load data for the 300gn TSX FN for guidance with the #30642 but realize that available case volume will change!

the following are all TSX bullets
#30629 250gn FN 1150fps ? 1600fps
#30615 300gn SP 1600fps ? 2300fps
#30630 300gn FN 1150fps ? 1600fps
#30617 350gn SP 1600fps ? 2300fps
#30619 450gn SP 1500fps ? 2200fps
#30622 500gn SP 1400fps ? 2200fps