The 5"/54 caliber Mark 16 gun (5-inch, 127 mm) was a late World War II–era naval artillery gun mount used by the United States Navy and Japan Maritime Self-Defense Force. These guns, designed specifically for the Montana-class battleships, were to be the replacement for the 5"/38 caliber secondary gun batteries then in widespread use with the US Navy.
The 5"/54 cal gun turrets were similar to the 5"/38 caliber gun mounts in that they were equally adept in an anti-aircraft role and for damaging smaller ships, but differed in that they weighed more, fired heavier rounds of ammunition, and resulted in faster crew fatigue than the 5"/38 cal guns.
The ammunition storage for the 5"/54 cal gun was 500 rounds per turret, and the guns could fire at targets nearly 26,000 yd (24 km) away at a 45° angle. At an 85° angle, the guns could hit an aerial target at over 50,000 ft (15,000 m).
The cancellation of the Montana-class battleships in 1943 pushed back the combat debut of the 5"/54 cal guns to 1945, when they were used aboard the US Navy's Midway-class aircraft carriers. The guns proved adequate for the carrier's air defense, but were gradually phased out of use by the carrier fleet because of their weight (rather than having the carrier defend itself by gunnery the task would be assigned to other surrounding ships within a carrier battle group).