6"/47 caliber gun

The 6"/47 caliber Mark 16 gun was used in the main batteries of several pre-war and World War II US Navy light cruisers. They were primarily mounted in triple turrets and used against surface targets. The 6"/47 caliber Mark 16DP gun was a dual purpose fitting of the Mark 16 for use against aircraft as well as surface ships. It was installed in the post-war light cruisers and the anti-aircraft gunnery training ship USS Mississippi (AG-128).

The 6"/47 caliber Mark 17 gun was a variation of the Mark 16 to use bagged charges; this was only used in the in a single pedestal mount.

Design
Three versions of this breech loading rifled naval gun were produced, the 6 in/47 Mark 16 Mod 0, the 6-inch/47 Mark 16 Mod 1, and 6-inch/47 Mark 17. "6-inch /47" refers to a bore diameter (caliber) of 6 inches and a bore length of 47 calibers (a caliber is one bore diameter), or 23 ft. "Mark 16" indicates it is the 16th design in the series of US Navy 6-inch guns. "Mod 0" or "Mod 1" indicates minor modifications to the design, with 0 being the original and 1 being the first modification (which featured a tapered liner).

The 6-inch/47 caliber gun was one of several weapons developed by the United States Navy in the 1930s to fire "super-heavy" armor-piercing (AP) projectiles, thus increasing warships' destructive power while complying with the limits on number of guns and ship size by the London Naval Treaty. Compared with the preceding 6-inch/53 caliber gun, the 6-inch/47 Mark 16 fired a 130 lb AP projectile instead of a 105 lb AP projectile.

The guns were mounted in three types of turret.

The Mark 16 was primarily mounted in a triple turret for use against surface targets. All three guns in each turret were mounted in the same sleeve and thus elevated together, but delay coils permitted "split salvos" to be fired; this cured a shell pattern dispersion problem common to many US cruisers of the 1920s and 1930s. The had 12 guns mounted in four triple turrets. The arrangement in triple turrets allowed the use of all guns in a broadside; the light cruisers of the 1920s mounted twelve 6-inch/53 guns but could only use eight in a broadside, due to most of the guns being in single casemated mounts.

The Mark 16DP used a two-gun semi-automatic "Dual Purpose" turret, for use against both air and surface targets. They were individually sleeved to allow independent elevation. They were produced in limited numbers late in the war. The DP turret could fire more quickly and elevate and train faster compared to the "single purpose" triple turret. The Worcester-class used these mountings. These were not entirely satisfactory, and a triple DP mounting was proposed to replace them, but was cancelled after World War II.

The Mark 16/16DP gun could fire a 130 lb projectile 11.36 mi at an elevation of 22.3 degrees with a flight time of 44.7 seconds. Maximum range at 44.5 degrees elevation was 14.77 mi with a flight time of 77.3 seconds. Projectiles varied in weight; an armor-piercing projectile weighed 130 pounds, while a high-capacity (HC) projectile weighed 105 pounds. Ammunition was semi-fixed (the projectile and the powder casing were separate). The full charge powder case for these guns was the Mark 4 housed in a brass canister and weighed 65 lb. The HC projectile could be equipped with mechanical time (MT) or, by late 1942, with variable time (VT) radio proximity fuzes for use against aircraft.

The Mark 34 high-explosive shell this gun fired is usually referred to as "HC", but, when fitted with a proximity (VT) fuse or a mechanical time (MT) fuse, it could be used against aircraft and thus was technically an "AA" projectile in that configuration. Thus the Mark 34 HC is also in theory the Mark 34 AA, depending on the fuse fitted.

Eight to ten rounds per minute could be fired from each of the 6-inch guns. Each gun weighed 6.5 ST and could originally only be evevated up to 40 degrees but were later modified to be elevated up to 60 degrees. Originally gun ports in the turret faces were cut to allow only 41 degrees elevation, though during World War II all triple 6-inch/47 gun ports were ordered to be modified to permit the full 60 degrees. The guns could only be loaded at between -5 degrees and 20 degrees elevation; this reduced the rate of fire when engaging distant surface targets or aircraft. The 105-pound Mark 34 HC shell fired at 2665 ft/s out to 23483 yds at 46.6 degrees; the 130-pound Mark 35 shell introduced just before World War II fired at 2500 ft/s at full charge and could penetrate out to 26000 yds at 44.5 degrees.

Gun barrel lives were 750 to 1050 full-charge rounds.

The Mark 17 was used in a single pedestal Mark 18 mount. The Mark 17 gun could fire a 105 lb Common shell (HE) projectile 19800 yd at an elevation of 20 degrees. Ammunition was bagged (the projectile and the powder bag were separate). The full charge powder bag for these guns weighed 34 or. The Erie-class mounted four guns in single pedestal mounts.

Five to eight rounds per minute could be fired from each of the 6-inch guns. Each gun weighed 5.24 ST and could be evevated from -10 degrees up to 20 degrees. The 105-pound Mark 28 Common shell fired at 2800 ft/s.

Gun barrel lives were 750 to 1050 full-charge rounds.

Mounts
The "Mark 16" designation refers to the gun being 16th in the 6-inch series of designs, not the turret the gun is mounted in. Smaller guns at that time had a Mark number for the type of mounting. In modern times the USN refers primarily to the Mark number of the gun mount (turret), but in WW II the model of the gun was the primary reference point. The gun turrets for most 6-inch and larger guns of the 1920s through WW II were known according to the class of ship the turret was to be mounted on.

A 6-inch triple turret weighed between 154 – in the and  cruisers and 165 – in the Cleveland-class and  cruisers, and each rifle barrel was 25 ft long. The turret rested on a barbette or circular shaft that extended several decks into the ship. Projectiles were stored in a projectile-handling room in the lower part of the barbette. Two-hundred projectiles, per gun, could be stored in the projectile-handling room. The guns were supplied with projectiles via hoists.

Powder stores were below the projectile-handling room and powder hoists fed the guns. Empty powder canisters were ejected from the turret via an ejector port at the back of the turret. When the guns were firing, it was not unusual to see empty brass canisters piling up on the deck behind the turret. The turret itself had 6.5 in of armor plate on its face and could train (turn) to follow its target at ten degrees a second.

Each turret required a crew of 3 officers and 52 enlisted men.

The Mark 17 guns were installed in the Mark 18 single pedestal mount that weighed 15.4 ST.

Weapons of comparable role, performance and era

 * BL 6 inch Mk XXIII naval gun : British equivalent light cruiser gun
 * 15 cm SK C/25 : German equivalent light cruiser gun but firing a lighter shell at higher velocity
 * 15 cm SK C/28 : approximate German equivalent
 * 152 mm /55 Italian naval gun Models 1934 and 1936 : approximate Italian equivalent
 * 15.5 cm/60 3rd Year Type naval gun : approximate Japanese equivalent

Surviving examples
Today one of the few 6-inch triple gun turrets left in the world is on the museum ship USS Little Rock (CG-4), which is located in the Buffalo and Erie County Naval & Military Park in Buffalo, New York.