Twin 40mm Gun

Starboard 40mm mount aboard the USS SLATER DE766

As commissioned in 1944, the SLATER carried one twin 40 mm Gun with MK51 director, located on the superstructure aft on the 01 level. A set of torpedo tubes were located on this deck just aft of the stack.

Shortly after her shakedown cruise in June 1944, it was decided to remove the torpedo tubes and augment the anti-aircraft battery by placing four more single army type 40 mm gun mounts in their place. Pictures of the SLATER's tour of duty as a target ship and convoy pictures show these four single mounts. In June 1945, prior to its Pacific tour, these four single mounts were replaced by two twin mounts with MK51 gun directors with lead computing sights located in tubs adjacent to the guns. These continual upgrades are a good example of how the Navy continually improved and up-gunned its ships to meet changing threats.

The original, single 40mm design was a product of the Swedish Bofors company and probably ranks as the most efficient close-in air defense weapon on any warship during WWII. 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, was contracted build these guns for US service.
The 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 license from Sweden to do so until June of 1941. The first twin Bofors was produced in January of 1942. A quadruple version followed in April. Bofors guns soon began appearing fleet-wide, though not until mid-1944 was demand satisfied (and not for the SLATER until May of 45).

The efficient 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.

The most effective countermeasure to the kamikaze turned out to be the VT or proximity fuse. In this ammunition, the projectile was detonated electronically. The 40mm was too small to handle the electronics of the proximity fuse, the 76mm guns being the smallest weapons able to handle that devastating projectile. Despite this, the Bofors 40mm remained the primary close-in anti-aircraft weapon of smaller vessels until the Japanese surrender.


Twin barrels of a 40mm gun aboard the USS SLATER DE766



40mm gun aboard the USS SLATER DE766 circa 1944


40mm gun aboard the USS SLATER DE766



STATISTICS:

Year of Construction: 1942
Bore: 40mm
Weight of gun: 522kg / 1150lbs
Weight of barrel: 91.6kg/ 202 lbs
Length of gun: 3,779.5mm/ 148.8 ins.
Length of bore: 2,250mm/ 88.853 ins. (56 calibers)
Wt. of projectile: 0.9kg / 1.985 lbs
Max. Range (Anti-Ship): 10.1km / 11,000 yds at 42° elevation
Max. Effective Range: 2.7km /3,000 yds
Ceiling (Anti-Air): 6.950 km/ 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)