USS San Juan
Specifications - (continued)
Ben Gross, son of San
Juan crew member Lynden B. Gross, provided the following information.
He is gathering specifications on the ship so he can build a realistic &
detailed model for his Dad.
Thank You Ben!
Oerlikon 2 cm/70 (0.79") Mark 1
United States of America
2 cm/70 (0.79") Marks 2, 3 & 4
Widely used by the Allied nations, the Swiss-designed 2 cm Oerlikon
automatic weapon was probably produced in higher numbers than any other AA weapon of WWII. The USA alone manufactured a total of
124,735 guns. The first USA built gun was test-fired on 8 June 1941 and 379 had been delivered by 7 December 1941.
These guns were introduced in the British Royal Navy in 1939. It is not specifically known how many guns were built by Britain and the
Dominion nations, but the Mounting Appropriation Lists of September 1945 shows about 55,000 guns in service in those navies. This total
probably includes USA built weapons provided as a part of Lend-Lease and on those ships that were refitted in US
This weapon proved very popular with its ease of maintenance and good rate of fire. They were air-cooled and used a gas blow-back
recoil system. In the USN, this weapon replaced the ineffective
0.50" (1.27 cm) BMG on a one-for-one basis and was the primary anti-aircraft gun until the Bofors 4 cm became available in large
numbers during 1943.
Between December 1941 and September 1944, 32% of all Japanese aircraft downed were credited to this weapon, with the high point
being 48.3% for the second half of 1942. In 1943 the revolutionary Mark 14 Gunsight was introduced which made these guns even more effective. This gunsight was developed by Dr. Charles Draper of
MIT, who calculated that since the guns fired at relatively short ranges, a crude but simple and effective relative-bearing system could be used to control these weapons. 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. Use of the Mark 14 did require that an electric power connection be provided to the formerly free-standing
mountings. This gunsight was later adopted as part of the Mark 51 director which was used to control the 4 cm Bofors, greatly
increasing their effectiveness. See the article on the Mark 51 director at our Technical Board for additional information.
Postwar, the Mark 14 was replaced by the Mark 20 Gun Sight, which was a lighter, simpler design. The Mark 20 was ready to use in ten
seconds while the Mark 14 took three minutes.
In 1944-45, the US found that the 2 cm shells were too light to stop Japanese Kamikaze planes and the higher approach speeds of these
planes made manually controlled guns obsolete. As a result, the 2 cm Oerlikons were replaced by 4 cm Bofors where possible during
1944-45 and removed entirely from most US ships by the mid 1950s.
Use by other nations: 1) The Italians purchased small numbers of this weapon directly from Oerlikon during World War II. 2) Some
2,002 of these guns were sent to the Soviet Union during World War II as part of Lend-Lease.
Some historical irony: Oerlikon almost went bankrupt in 1935 when the USN rejected their 2 cm Model 1934 weapon because of its low
rate of fire (265 rpm). Only the Imperial Japanese Navy's purchase of this weapon saved the company and permitted further development
work which resulted in the much more successful model used during WWII.
The Mark 1 was the original design by Oerlikon. A small quantity of this version was built in the USA as prototypes and possibly in
Britain, as well. The USA Mark 2 and the British Mark II were the first production versions in those countries. The differences from
the Mark 1 were mainly in the arrangements of the buffer springs although the USA Mark 2 also had cooling ribs and 2 locking slots.
The Mark 3 was similar to the Mark 2 but had fewer cooling ribs and only 1 locking slot. The later US Mark 4 had a single, heavier
buffer spring and was built to slightly different tolerances as it was designed using English units rather than metric units. The Mark
4 was the most common version in USN service. The Mark 1 could be fired in single-shot mode, while all of the others could only be
fired in automatic mode. All guns used a monobloc barrel and a horizontal sliding breech block mechanism.
This weapon has some unusual features not found in other automatic weapons. The barrel does not recoil, the breechblock is never
locked against the breech and is actually moving forward when the
gun fires, and there is no counter-recoil brake, the force of the counter-recoil is checked by the explosion of the next round of
2 cm/70 (0.79") Mark 2 on USS Iowa BB-61
Note the Mark 14 Gunsight
Detail from U.S. Naval Historical Center Photograph # K-16469
Bofors 4 cm/60 Model 1936
United States of America
4 cm/56 Mark 1 and Mark 2
One of the best heavy MG AA weapons of WWII, the Bofors 4 cm/60 Model 1936 is still in service in some nations. This weapon was
used on almost every US and UK warship of WWII and was a very potent AA gun. The Germans also used the Bofors design to develop the 4
cm/56 Flak 28 and the Japanese copied a British Army type Bofors captured at Singapore to produce the 4 cm/60 Type 5.
This weapon traces its roots back to a 1918 Krupp design - the Bofors Company was owned by German interests until 1930. What
became the 4 cm Bofors was first developed in 1933 but it was the
Model 1936 that was adopted for production. The British Army placed an order for 100 of these weapons in 1937. First Royal Navy
shipboard use was in late 1941 - although some army guns were
"rescued" and mounted on ships evacuating the 1940 Norwegian invasion forces. These were all air-cooled guns. The British
water-cooled version was developed from the Dutch Hazemeyer
mounting. This weapon arrived in Britain in 1940 aboard the Dutch minelayer Willem van der Zaan. The first issue of the locally
produced water-cooled guns to British ships was to HMS Whimbrel in November 1942. The total number of air-cooled guns built by
Australia, Britain and Canada is not known but was approximately 2,100. Water-cooled guns are better documented and there were 442
Mark IV and 342 Mark XI in service. The Royal Navy was also supplied with about 167 air-cooled and 786 water-cooled guns from
the USA when ships were refitted in the USA or as part of Lend-Lease.
In the USA, there was great pre-war interest in this weapon and BuOrd purchased a sample of the air-cooled twin version, which
arrived in New York from Sweden on 28 August 1940. During the same month, the Dutch escort vessel van Kinsbergen demonstrated these weapons to US observers in a test off Trinidad. BuOrd formally
obtained Swedish licenses in June 1941, although some manufacturing actually started prior to that time. The first pilot twin was produced in January 1942 and the first quad in April 1942. The
first shipboard installation was on the gunnery-training ship (ex-battleship) USS Wyoming (AG-17) on 22 June 1942, followed by USS
Coghlan (DD-606) on 1 July 1942. The USA started a massive production program for these weapons, with a total of about 39,200
weapons being built in the USA alone. Even so, the demand was not fully met until well into 1944. The Gridley class (DD-380)
destroyers were the only first-line destroyers in the USN not to receive this weapon.
All early versions of this weapon used friction-coupled drives, which rapidly wore out due to salt contamination. Later versions built in the USA used hydraulic-coupled drives which eliminated the
problem. USN single mounts were air cooled, naval twin and quadruple mounts were water cooled.
The development of the Mark 51 director system gave the USA weapons greatly improved accuracy. For example, half of all Japanese
aircraft shot down between 1 October 1944 and 1 February 1945 were credited to the Bofors/Mark 51 combination. See the article on the
Mark 51 director at our Technical Board.
A modification kit is currently available to increase the rate of fire to 180 rounds per minute and the magazine capacity to 20 rounds
in a banana feeder fed by standard 4 round clips. Unless otherwise noted, the data that follows is for the USA Mark 1
and Mark 2 versions, but the weapons built by other nations had similar performance. Mark 1 is a left-hand weapon and the Mark 2 is
a right-hand weapon. These weapons could be fired in single-shot or automatic mode via a selector switch on the side of the slide.
1.1"/75 (28 mm) AA MG Mark 1
This water-cooled heavy AA MG was widely used on US warships before
WWII. It was found to be unreliable in early service use, prone to jamming and ineffective as an AA gun. They were rapidly replaced by
the 2 cm Oerlikon and 4 cm Bofors AA weapons during WWII and by 1945 only survived in a few smaller ships.
In retrospect, it would appear that these defects were little more than teething problems and that the basic design was sound.
However, it still lacked the range and larger bursting charge of the 4 cm Bofors. All surviving guns and mounts were ordered scrapped in
1945 but some of the power drives were retained as being suitable for the twin Bofors mountings.
Used a monobloc barrel with chromium plating both internally and externally. Mod 1 had a threaded barrel to accept a flash hider.
Mark 2 was an experimental gas operated version that did not enter service.
1.1" (28 mm) Quad Mount
The guns in the background are 2 cm Oerlikons
San Juan Specs -