I measured the magazine box, it is a "tight" 3-7/16" inside length. And it is wider at the front than the box in my Mark X 300 Win Mag. Rails on the 458 are also wider at the front than the rails on the 300 Win Mag. So, yes, as said above, go slow and open up the feed rails, and give it a nice U-shaped ramp. Slow, and lots of trying with dummy rounds.