I was just in Baltimore, and something stood out about the Coast Guard ship on display there. Its armed with a pretty serious gun, its not the USS Arizona butthis gunis not the kind machine gun which (I imagine) is the largest weapon you would consider using in the kind of police actions the Coast Guard is usually involved in.
I don’t know if the “in anger” part applies, but a Coast Guard ship did fire its main gun with the intention of sinking a ship a few years ago. The ship was an abandoned Japanese fishing vessel that had broken free during a tsunami. The ship drifted for about a year, finally reaching U.S. territory. Since the derelict ship was becoming a hazard to local shipping routes, the Coast Guard sunk it.
Main gun (of the 110’ patrol boat Anacapa) but only a 25mm so I don’t know whether it’s what OP has in mind asking about USCGC Taney (the museum ship in Baltimore) which has a 5" (127mm) main gun(s, 4 of them late in WWII, only one postwar).
More recently USCGC Sherman sank a derelict US fishing vessel, but used the secondary and tertiary armament (25mm and .50 cal) not the main 76mm gun.
The Coast Guard last fired 5" guns in combat in Vietnam. The pre WWII (Campbell or 327’ class of which Taney is a member) and WWII ships with 5" were still around and the then-new 378’ class ships also had 5"/38 guns recycled from WWII ships. They often fired in support of forces ashore and at least once at a North Vietnamese blockade runner. Taney last fired the 5" on a tour off Vietnam in 1969. Since then the 378’ers, now being retired, were refitted with more modern 76mm guns. Their replacements carry 57mm main guns. I don’t believe any of the ‘big’ post 5" guns have ever fired ‘in anger’.
U.S. Battleship Pounds Hills Held by Syrians in Lebanon; Britain; Pulling Out Troops, from the February 9, 1984 New York Times, reported that “The United States battleship New Jersey bombarded Druse and Syrian gun batteries in Lebanon for more than nine hours today in the heaviest and most sustained American military action since the marines arrived here 16 months ago.” and “In Washington, the Pentagon said the destroyer Caron joined in the bombardment, firing more than 300 five-inch shells. It said the New Jersey fired more than 250 16-inch shells.”
I believe that the battleship was decommissioned shortly after this, with more modern munitions, especially cruise missiles, making it obsolete.
Thanks that answers my question. Always strange to me that the US Coast Guard is a part of the US military that operates a long way from the US coast its meant to be guarding (especially as the British equivalent is not only not part of the military, its actually a charity rather than part of the British government).