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Twin & Single 40mm Bofers AA Gun  


Bore 40mm
Weight of gun 1150lbs
Weight of barrel 202 lbs
Length of gun 148.8 ins.
Length of bore 88.853 ins. (56 calibers)
Wt. of projectile 1.985 lbs
Max. Range (Anti-Ship) 11,000 yds at 42 elevation
Max. Effective Range 3,000 yds
Ceiling (Anti-Air) 22,800 ft./ 7,600 yds at 90 elevation
Max. Elevation 90
Rate of Fire 120 - 160 rounds/min/barrel; governed by the speed of loading 4 round clips.
Crew Twin - 7; Pointer, Trainer, Gun Captain, 4 Loaders (2/Barrel)

        One of the single 40's aboard LST 325.
The 40mm AA mount, a product of the Swedish Bofors company, was an  efficient close-in air defense weapon during WWII. The gun was first developed in 1933 but it was the Model 1936 that was adopted for production.  The British Army placed an order for 100 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.  A single--barrelled air-cooled version was first examined by the U.S. Army in 1937. In 1940 the Chrysler Corporation, one of America's "big three" auto manufacturers, agreed to build them using British drawings.  

The USN acquired some of these, mounting them in submarines, destroyers, some lighter vessels and LST's.  These USN single mounts were air cooled.   Later in 1940, a twin-barrelled example arrived from Sweden via Finland and, using British and Dutch plans, the York Lock & Safe Co. undertook to manufacture them for the Navy, though it did not receive formal licence from Sweden to do so until June of 1941. The first twin Bofors was produced in January of 1942.  The naval twin and quadruple mounts were water cooled.  Total USA production was about 39,200 weapons. The 40mm proved adequate for all close-in air defense duties until the kamikaze attacks of the last nine months of the war. Each of the twin 40mm's fired 160 rounds-per-minute, per barrel with an approximate effective range of 4000 yards. 

The guns were manned by seven men with ammunition passers added to the crew as required. The crew consisted of a gun captain, pointer, trainer, two first loaders and two second loaders. Ready service ammunition was held in racks welded to the "gun tub", a steel bulwark built around the gun to protect the crew. When ammunition in the ready service racks was used up, they would be refilled from below. Despite all this, it was not devastating enough to stop many diving suicide planes from crashing into their targets. Part of the problem was that the 40mm couldn't handle the VT proximity fuse, the 76mm guns being the smallest weapons able to handle that devastating item. Even still, the Bofors 40mm remained the primary close-in anti-aircraft weapon of smaller vessels until the Japanese surrender.

The development of the Mark 51 director system gave the USA weapons greatly improved accuracy. 


Gun   Gun Numbers   At Each Gun
Twin 40 mm 1 328 rounds
Twin 40 mm 6 420 rounds
Single 40 mm  2-5 inclusive 276 rounds


 Number of rounds per clip 4
Number of clips per box  4
Number of rounds per box 16
Weight of projectile, complete 1.96 lbs
Weight of round (brass case) 4.75 lbs.
Weight of clip (assembled) 20 lbs.
Weight of box, loaded, 16 rounds   115 lbs.
Dimensions of box 12 x 12 x 22
Tracer burnout  9 seconds
Condition of ammunition as received  Clipped, ready for use
Fuse Bore safe (armed by rotation).  Impact detonated self-destruction (operated on tracer burnout)


20 mm Oerlikon AA Gun

Bore 20mm
Weight of gun 141-150 lbs
Weight of barrel 48 lbs
Length of gun 87 ins
Length of bore 55.118 ins
Wt. of projectile varies .27 lbs -.53lbs
Max. Range  4800 yds
Max. Effective Range 1000 yds
Ceiling (Anti-Air) 10,000 ft at  90
Max. Elevation 90
Rate of Fire varies 265 rounds/minute (early model) - 650 rounds/minute
Magazine 60 round drum
Crew 5 to 7 (Gunner, Trunnion Operator, Range(Sight) Setter if gyroscopic sight, Loader) Gun is aimed and elevated manually.

The Bureau of Ordenance favored a free-swinging anti-aircraft gun, rather than the slower powered mounts available at the start of the war, which could be used even without any source of power.  The only current free-swinging weapon was the .50 caliber machine gun, which was lacking in range and firepower. A replacement was needed that had firepower but still with a low enough weight to be manually handled.

Widely used by the Allied nations, the Swiss-designed 2 cm Oerlikon AA MG was an outstanding candidate.  It 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.  Earlier the gun had been reviewed for service, but rejected on grounds of low rate of fire. Eventual adoption of the 20mm may have been due to the British who had lost their contact with Switzerland in 1940 and wanted a U.S. production line. Eventually, on the grounds of it being the best available weapon, BuOrd approved the production in November 1940.

Reception was enthusiastic as a more than welcome replacement for the .50 caliber, production was slow to quickly replace all of the .50 Cal, so in the early months most air-defenses continued to rely on the .50-caliber.

Ammunition was held in sixty round drums that fitted on top of the breech. After long periods of firing (about 240 rounds) the barrels had to be changed by the loaders, who wore asbestos gloves. The hot barrels were switched with spare barrels stored in cooling tanks welded alongside the guns. The ammunition drums were stored in lockers behind the gun mounts. When first placed in service in 1942, these guns were mounted singly and were aimed by open ring sights. After 1942, the lead computing stabilized MK14 relative rate gunsight was mounted on these guns for better accuracy, and added the sightrange setter. In 1945, the twin mount became common, which added two more loaders.

The career of the 20mm was quite short. It had extraordinary successes in 1942, 1943 and up to mid-1944, but with the advent of the kamikaze, the 20mm was not adequate anymore. It was, however, retained because of the psychological effect that the ability to fire at the attacker had for the seamen, and twin and quadruple mounts had been tested and installed aboard several ships.

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.

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.

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 Japanese Navy's purchase of this weapon saved the company and permitted it to perform further development work which resulted in the much more successful model used during WWII.


 Number of rounds per magazine 60
Number of rounds per box  180
Weight of projectile .27 lbs
Weight of round   .58 lbs
Weight of magazine (unloaded) 3.2 lbs.
Weight of magazine (loaded)  37 lbs.
Weight of box (loaded)  120 lbs.
Dimensions of box 18 x 9-1/2 x 12-1/2
Tracer ratio 1 tracer, 2 non-tracers
Tracer burnout 3-3/4 seconds
Condition of ammunition as received In paper tubes--must be greased
Fuse Not bore safe (except BL-P). Impact detonated, no self-destruction feature


Sources:  Some of the info contained in this page was found in the following web sites.  Please visit them!