[akdavis]

Here is another pretty definitive source, a USN 1957 manual:

3D7. Penetrating projectiles

This type includes armor-piercing (AP) and common (Com). They are designed to penetrate, respectively, heavy and light armor. The usual bursting charge for these types is Explosive D, which is insensitive enough to permit penetration without premature detonation. The characteristics which make that possible will be described under the heading of penetration in the next chapter.

3D8. Fragmenting projectiles

These projectiles are designed to inflict damage both by blast effect and by fragmentation; that is, breaking up into small high-velocity fragments. They are characterized by thin walls and large cavities for the explosive filled. The general type is subdivided as follows:

1. High-capacity (HC) projectiles (fig. 3D3) are used against unarmored surface targets, shore objectives, or personnel. Since no penetration ability is required, explosives more sensitive than Explosive D may be used.

2. Antiaircraft (AA) projectiles are designed for use against airplanes in flight. Except for fusing they are substantially the same as high-capacity in the larger calibers. In smaller sizes the explosive often contains an incendiary element.

3. Antiaircraft common (AAC) projectiles are a dual-purpose design, combining the qualities of antiaircraft projectiles with the toughness necessary to penetrate steel plating not of armor thickness. The type of fuzing will depend on the use. The walls may be heavier than those of the other thin-walled types, and the filler is usually Explosive D.