I just purchased some Barnes .458 originals. Part of the deal is a box of the 300 grain flat point. Anyone here used them in the 45-70 or 450? I'm a bit concerned about minimal expansion at 2100fps muzzle velocity.

I have used them and the semi-spitzer in both 450 Marlin and 458 Win Mag. The FN is stouter than a Sierra but still expands down to 17-1800fps. As long as your shots are not too long you should be ok.