The way my dear old dad put it was, "Never be afraid to ask a stupid question. They're easier to deal with than stupid mistakes."

The term "nitro" simply refers to smokeless powder. If the cartrdige is designated as XXX-XXX Nitro Express it was developed for smokelss powder. If it is designated XXX-XXX Nitro for BP it is a smokeless loading in a cartridge orginally developed for black powder and developes a safe pressure for the black powder fire arm.

While we are at it, the term "express" means a rifle cartridge that developes a muzzle velocity in excess of 1800 fps or so. A real trick with black powder when the term was coined but easy to do with smokeless.

The term "magnum" means a cartridge with a larger than standard powder capacity for cartrdiges in the same caliber and class. Thus the 375 H&H (For which the term was coined.) is a magnum since the powder capcity is larger than any 375 that preceded it. On the other hand the 458 Winchester Magnum and the 32 H&R magnum are not true magnums since they have LESS than standard powder capcity for cartridges in their class. The 458 has the smallest powder capacity of almost any 45 except the 45-70 and the new 450 Marlin. The 32 H&R has a shorter case than any magnum revolver cartrdige and has less powder capacity than the 32-20. Hence the "magnum" moniker is pure advertising copy writer's hyperbole.

BTW: The reason the 458 Winchester has such a small case is so that it can be used in a standard length bolt action.