That's a funny thing; designations from different navies are so, well, different.
In the Royal Navy a 'Common' shell was also called 'Semi-Armor Piercing' (SAP) and was expected to penetrate armor equivalent to 1/3 the shell's diameter. A book I saw once on Fletcher class destroyers made the same claim for US 'HC'. One of the funnier things about the British designations was the brief use of 'SAPCBC', or 'Semi-Armor Piercing Capped British Common'.
Another reference I've seen to 'HC' was that it was designed for shore bombardment, and was meant to penetrate xx amout of concrete. Sorry, but I don't remember the actual amount.
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