Ray,
Regarding compaction, I just loaded some 458 Winchester rounds with 73 grains of AA2230 and 450 grain Barnes TSXs, WW cases. COL was 3.34 inches, which is what is recommended.
With this load there is no compaction. Powder density is just about 100%. With this load I get almost 2300fps and comfortably over 5000 foot pounds of energy.
I therefore have to agree with some others: The .458 Magnum is indeed an underrated cartridge.
