Speer 350gr. .458 has a 1/16" thick jacket & is meant for the .458 Winchester.
For any expansion to take place, it must be started at a minimum of 2,100fps, seems to me. Might have been 2,000fps minimum MV.
They were very accurate in my .458 2", at 2,300fps. Sold my last rifle in this calibre, before getting to shoot anything with them besides paper.