I have owned 4 458s at different times. The first was about 1970 and was a M70 with the 22 inch barrel. Over the Oehler 10 (the first Oehlers with paper screens) the 500 grain FMJ Winchester factory load did 1960 f/s. I remember the velocity well because I got the Oehler just after the 458 and remember the velocities of different calibres I first chronographed.

As a side note that Oehler cost $175 which at the time was about the same price as a Sako in 270.

Feeding with straight sided cases and with blunt bullets is always less reliable than with spitzers or with bottle neck calibres. With blunt bullets something like a 458 only needs to be slightly off line and the bullet will catch the edge of the chamber.