I have read the problems with the .458 factory ammo developing sub-standard ballistics stemmed from the powder used by Winchester, being likely a non-production/proprietorial 748-type powder- old military ball or spherical powder. In order to get the published ballistics of 2,150fps from the requisite 25" barrel, they needed a very heavily (powder) compressed charge of it.

Since this WW .458 factory ammunition was stored in Africa, the storage of it lacked the fundamental rules we now know, of cool, dry storage for the longevity (long-life) of the product.

Hot and dry, or hot and wet in Africa, are not synonymous with longevity of powders, smokeless nor black.

The powder in the .458 magnum factory rounds, being heavily compressed in these cartridges & stored under such arduous conditions, became caked, virtually a solid mass thus did not burn correctly, sometimes failing to go off at all. Oft times, it developed sub-standard ballistics as John noted. These failures did the damage which today has put a black mark on the .458 Winchester Magnum.

There are very good powders today that do not share this caking and poor function that the OLD ammunition was plagued with.

Today, with modern powders, the .458 does everything it was ever meant to do.