U.S.S. LITTLE ROCK CLG-4 / CG-4
20mm/70 Gun

Last Updated: 24 September, 2016


GENERAL INFORMATION

20 mm/70 (0.79") Marks 2, 3 & 4

The 20 mm gun was originally designed by the Swiss firm Oerlikon, and  was most likely produced in higher numbers than any other anti-aircraft weapon of World War II.

In 1935 the US Navy purchased two Model 1934 Oerlikons for evaluation purposes, but rejected this model because of it having a low muzzle velocity and slow rate of fire. Ironically Oerlikon almost went bankrupt when the USN rejected the Model 1934.  Only the Japanese Navy's purchase of licensing rights saved the company, allowing further development work, which resulted in the much more successful versions used by the Allies during World War II.

After evaluation of alternative guns, BuOrd's Capt. W.H.P. Blandy recommended adoption of the improved Swiss-built Oerlikon Mark I. The first Oerlikon made in the US was test fired on 8 Jun 41, and by 7 Dec. 41, a total of 379 weapons had been produced.  Stimulated by the start of the Pacific War a total of 124,735 US manufactured Oerlikon guns (Mark 2, 3, 4) were produced before production ended in 1945.

The 20 mm Oerlikon, with its ease of maintenance and good rate of fire was the Navy's primary AA until the 40 mm became available in large numbers in 1943. Between Dec 1941 and Sep 1944, 32% of all Japanese aircraft downed by the USN were credited to this weapon. In 1943 the new Mark 14 Gunsight, developed by Dr. Charles Draper of MIT, was introduced making these guns even more effective.  Draper calculated that since the guns fired at relatively short ranges, a simpler but more effective relative-bearing system could be used to control the gun. The Mark 14 gunsight used two gyros to measure vertical and lateral rate of change, calculated the lead angle to the target aircraft, and then projected an off-set aiming point for the gunner.

In 1944-45, the Navy found 20 mm shells were too light to stop Kamikaze planes, and the higher approach speeds of these planes made the manually controlled 20 mm guns obsolete.  Consequently 20 mm Oerlikons were replaced by 40 mm Bofors, and the Mark 14 gunsight used so effectively on the 20 mm was adopted as part of the Mark 51 fire control director used to control the 40 mm Bofors.




The following is extracted from:

NAVAL ORDNANCE AND GUNNERY, VOLUME 1, CHAPTER 9, "AUTOMATIC WEAPONS"

B. 20-mm Aircraft Gun

9B1. (See Reference link below.)

9B2. (See Reference link below.)


9B3. Development of the 20-mm Gun M3

During the interim between World War I and World War II, both the Germans and the Japanese developed 20-mm guns for aircraft installation, and during World War II the Germans installed as many as six of these guns in some planes. Our own development of a 20-mm gun was initiated in 1937, and speeded up when the European conflict began.

Since its inception this gun has undergone two major changes, each incorporating some improvement in design and operating characteristics. The end product of this technological advance is the 20-mm Aircraft Gun M3.

9B4. General description

The 20-mm Aircraft Automatic Gun M3 is an air-cooled weapon weighing approximately 100 pounds, and is capable of firing up to 800 rounds of ammunition per minute at an initial velocity of 2,730 feet per second.

Operating with the gun is a feed mechanism mounted on top of the gun proper. Rounds of ammunition are fed into the gun from a disintegrating link belt. A charger retracts the breechblock initially and cocks the gun, or can remove a round from the chamber. Firing is done by an electric trigger, which fires the 20-mm Automatic Gun M3 by remote control.

9B5. Principles of functioning (See Reference link below.)

9B6. Recoil mechanism (See Reference link below.)

9B7. Receiver (See Reference link below.)



THE 20 MM GUN USED ON THE U.S.S. LITTLE ROCK

Dual 20mm gun

NAVAL GUN TERMINOLOGY and DEFINITIONS

For interested non-Gunnersmates, CLICK HERE to go to a short list of terms and definitions used in Naval Gun discussions and literature.
REFERENCE LINKS

NAVAL ORDNANCE AND GUNNERY MANUAL
(Provided by Gene L. Slover)

NavWeaps.com Naval Gun Page

Click HERE to link to a copy of the 20 MM Antiaircraft Gun, OP 911, 1943

Click HERE to link to a copy of the 20 MM Antiaircraft Gun Mount, OP 909, 1943


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