[akdavis]

I noticed that along with the 3" and 5" guns, the 4"/50 now only one type of ammunition, but I believe fleet boats carried a mix of HC and Common rounds for the 4" guns, as is evidenced in a number of patrol reports:

Razorback

...50 rounds of 4" H.C. and common were fired...

Batfish

Four-inch gun. - Fired 59 rounds of high capacity and 24 rounds of common at an average range of about 1100 yards using radar ranges and radar spotting on the splashes. Local control at the gun. Estimate at least 40% hits. The HC was much more effective than the common as the latter passed right on through the target and unless it hit a vital spot did no immediate damage. Estimate 5 HC duds.

Archerfish

Damaged - One 250 ton trawler-typePatrol Boat: Two Masts, Bridge Structure Forward of MOT

Damage determined by observation; six hits with high explosive 4" rounds, approx. 250 rounds of 20 mm and 50 cal hit target. (Both masts were destroyed and superficial damage to bridge works and top side was observed)

Details of Action

Gun Rounds Hits Average Range
4" 50 cal 60 H.E. 6 4000 yds
4" 50 cal 60 Common 0 4000 yds

Common is basically AP/HE ammo, with thicker walls than HC but a smaller bursting charge (about 50% of the HC charge). They were not true AP shells, but were designed to penetrate 1/3 of their caliber in armor plate (so a 4" Common round would penetrate 1 1/3 inches of armor).

Navweapons.com:

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.

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.