Here is the real story regarding the .458 win mag. To get 2150 fps with a 500 grain bullet extruded powder under heavy compression was required. The chemical composition and coatings caused the powder to become a solid over time (as little as a year). This had detrimental effect on velocities as one can imagine.

Today's “modern” powders will do exactly the same if you compress it. I recently pulled some two year old compressed .375 loads and had to chisel the powder out!

Compressed loads are OK if you shoot them in the next 30 to 90 days. Some of you are now saying hog-wash, but this has be confirmed by two factory loaders I contacted for custom ammo.