I tagged a moose with one of those 350 Speers a few years ago. Animal was close - probably 20 yards or so and the bullet slipped in from the front running just through/behind the shoulder. It stopped somewhere in the 100-plus pound stomach leaving a 3/4" hole as it's last evidence. The animal spun and ran off a few yards before tipping over. Bullet impact speed was probably around 2100 fps.
They're the hardest/toughest cup/core bullet for the 458 cal I've used. However they still throw their jackets like any cup/core can under duress.
