Thanks, Nisgeis. My assumptions about the AA guns was correct, though I was off on the deck guns. Navweaps gives the following descriptions of H.C. and Common ammo:
First, H.C.:
Quote:
HC - High Capacity. A USN designation for projectiles intended for use against lightly armored targets. Contains a relatively large amount of explosive as compared to an armor piercing or common projectile. Burster was between 7.0% to 12.6% of total projectile weight.
HE - High Explosive or High Effect. Same as HC.
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And then Common:
Quote:
Common - Common projectiles were originally shells - which literally means a hollow container - filled with black powder and used for attacking lightly armored or unarmored vessels. By the 1930s, this term was used by a few navies to describe any non-armor piercing shell. By that time, the bursters were less sensitive explosives, such as TNT. In the USN, Common projectiles of the 1920-1950 period did not have caps or hoods and were designed to penetrate approximately one-third of their caliber of armor. See "Special Common" below.
Special Common - USN unofficial designation of the 1920-1950 period for those Common projectiles that used both windshields and hoods. These projectiles were designed be able to penetrate approximately one-third to one-half their caliber of armor. They differed from AP projectiles by not having a cap and having a larger burster cavity. Burster was 2.1% to 3.99% of total shell weight.
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Based on that info, it looks like both rounds could be considered HE, especially considering the statements about Common in that
"it differed from AP projectiles" and
"it was used...to describe any non-armor piercing shell."