1.1"/75 caliber gun

1.1"/75 caliber gun
Public domain · source
Name	1.1"/75 caliber gun
Origin	United States
Type	Naval anti-aircraft gun
Service	1939–1945
Used by	United States Navy, United States Marine Corps
Wars	World War II, Pacific War, Battle of the Coral Sea, Battle of Midway
Designer	Bureau of Ordnance
Design date	1930s
Manufacturer	Bureau of Ordnance contractors
Caliber	1.1 inch (28 mm)
Rate	150–300 rounds per minute (practical)
Feed	5- to 12-round magazines (per barrel)

1.1"/75 caliber gun

The 1.1"/75 caliber gun was a United States naval anti-aircraft autocannon developed in the 1920s–1930s and fielded on surface ships and small craft during the early years of World War II. It aimed to provide close-range air defense for United States Navy vessels and United States Marine Corps units, and saw action in major Pacific engagements such as the Battle of the Coral Sea and the Battle of Midway. The weapon's development, mounts, ammunition, and combat performance influenced later rapid-fire designs including the Bofors 40 mm gun and the Oerlikon 20 mm cannon installations widely adopted by Allied navies.

Development and Design

The 1.1"/75 originated from requirements issued by the Bureau of Ordnance to counter modern aircraft threats encountered by the United States Navy in the interwar period, with design work involving contractors linked to Naval Research Laboratory recommendations and inputs from fleet commanders following exercises near Guantanamo Bay Naval Base. Engineers sought a higher-velocity, larger-caliber alternative to the earlier Lewis gun-derived systems and the heavier 5"/38 caliber gun dual-purpose mounts, favoring a 75-caliber barrel length to optimize muzzle velocity and shell performance. Prototypes underwent trials at the Naval Proving Ground and aboard ships assigned to the Battle Force and the Scouting Force, with feedback from officers who had served in the Asiatic Fleet and the Atlantic Fleet. Design compromises included a complex feed system and an autocannon action influenced by earlier automatic weapons developed during programs overseen by Admiral Harold R. Stark and technical advisors from the War Department.

Technical Specifications

Standard specifications for the 1.1"/75 included a 1.1-inch (28 mm) bore, a barrel length of 75 calibers, and a theoretical cyclic rate advertised by the Bureau of Ordnance exceeding 300 rounds per minute per gun. Practical sustained rates dropped because of magazine changes and overheating under sustained fire, documented in reports from the Pacific Fleet and trials at Pearl Harbor. Mounting weights and stabilization requirements were influenced by ship classes such as the Porter-class destroyer, Yorktown-class aircraft carrier, and Clemson-class destroyer, with power requirements and crew drills informed by doctrine from Naval War College instructors. The feed employed small expendable magazines developed by subcontractors associated with General Electric and other ordnance firms; maintenance cycles were outlined in fleet technical manuals produced by the Bureau of Ships.

Operational History

The 1.1"/75 entered service in the late 1930s and was widely installed prior to World War II on United States Navy cruisers, carriers, and some destroyer escorts as part of prewar refits ordered by Admiral William Halsey Jr. and Admiral Chester W. Nimitz's commands. It saw extensive action during early Pacific campaigns, including the Philippine campaign (1941–42), the Battle of the Coral Sea, and the Battle of Midway, where crews noted both strengths and limitations under kamikaze and dive-bomber attacks. After action assessments conducted by boards including personnel from the Office of the Chief of Naval Operations recommended replacements following analyses shared with British Admiralty liaisons and Allied ordnance committees.

Mountings and Variants

Mountings ranged from single pedestal mounts on gun tubs to twin and triple enclosed power-assisted mounts aboard carriers and cruisers, with specific fittings produced for classes such as Yorktown-class aircraft carrier and Portland-class cruiser. Variants included training mounts used at shore stations like Naval Station Norfolk and experimental stabilized versions trialed by the Bureau of Ships in cooperation with Naval Aircraft Factory engineers. Field modifications by ship crews in theaters such as Guadalcanal produced improvised shields and altered feed arrangements, a practice also seen with other weapons on USS Enterprise (CV-6) and USS Hornet (CV-8).

Ammunition and Ballistics

Ammunition types for the 1.1"/75 included armor-piercing, high-explosive, and tracer rounds developed to engage fast aircraft and lightly armored surface targets, produced under contract by industrial firms linked to DuPont and other ordnance manufacturers. Ballistic performance emphasized muzzle velocity and effective ceiling in anti-aircraft engagement envelopes; range tables circulated through the Fleet Gunnery School compared the 1.1"/75's trajectory to that of competing systems such as the Bofors 40 mm gun and the Oerlikon 20 mm cannon. Logistic issues with magazine capacity and specialized propellant charges affected sustained engagement doctrine promulgated by officers trained at United States Naval Academy and tested during fleet exercises in the Caribbean Sea.

Effectiveness and Combat Evaluation

Combat evaluations after early Pacific battles criticized the 1.1"/75 for mechanical complexity, feed reliability, and comparative lack of stopping power against kamikaze aircraft, findings echoed in after-action reports from commanders of Task Force 16 and Task Force 17. The weapon's limited success accelerated adoption of the Bofors 40 mm gun and increased numbers of Oerlikon 20 mm cannon mounts under emergency procurement measures directed by the War Production Board and coordinated with Allied procurement through the Combined Chiefs of Staff. Some shipboard crews praised the gun's effective short-range barrage when properly maintained, a nuance reflected in evaluations archived by the Naval Historical Center.

Surviving Examples and Legacy

Surviving examples of the 1.1"/75 are preserved in naval museums including collections associated with National Museum of the United States Navy and regional institutions near Pearl Harbor National Memorial and Naval Station Newport, where artifacts are displayed alongside other World War II ordnance like the 5-inch/38-caliber gun and captured Type 96 25 mm AT/AA gun. The gun's operational lessons influenced postwar anti-aircraft doctrine and the development of automated systems such as those fielded by the United States Navy during the Cold War, and its service is documented in studies by naval historians from institutions including the Naval War College and the Smithsonian Institution.

Category:Naval guns of the United States Category:World War II naval weapons of the United States