> ...of a 422-grain copper bullet...
> The shot was made with a custom-built rifle chambered for the .416 Barrett cartridge
That's a huge bullet that was specifically designed for low-supersonic drag and extreme long range. It has a huge amount of energy at the muzzle... (8,767.4 ft-lb or 11,887 J according to wikipedia[1]) so ~600fps down range, even at 4.4 miles, is not that surprising.
The only surprising feat here is they managed to actually hit a target... which is quite incredible.