Shawn-
Armstrong guns are out of the picture for 1863 [indeed, until '64 or even '65], except, Loring is being quite hypothetical here and might have meant "Armstrong." But if so, why didn't he simply use the word, rather than the curious "usually used by the Navy"? Neither does he say "rifled" guns, so he may have meant guns of 3.67" & 4.58" bore (or if he presumed the reader knew he meant rifles, then ca. 2.5" & ca. 3"). To me it seems like nothing in inventory -- north or south -- fits what Loring is trying to describe. The "Clay" (apparently only one) was tried during the Seven Days and rejected, but showed up again in 1865 (late '64?) guarding the High Bridge west of Petersburg, not with the CS Navy.
John