Secondary Armament - Light AA Guns

While the heavy AA guns and their dual-purpose successors could be potent weapons against aircraft in the right situation, there were also cases where their limits were sharply revealed. Even the fastest-firing of them was limited to no more than one round every three seconds or so, and when the target was a strafing fighter or a torpedo bomber closing in for the drop, they simply could not respond fast enough. Smaller weapons, of limited range but capable of filling the air with steel, were needed.

Octuple Pom-Pom on HMS Rodney

Initially, these were simply versions of the standard machine guns used ashore, fitted with ring sights and bolted to a convenient railing. These were adequate in World War I, but shortly after the end of the war, the British saw the need for something better. They had also made use of the 2-pdr Pom-Pom,¹ a 40mm autocannon, for AA firepower and decided to base their future light AA weapon on it. Development began in 1920, but lack of funding slowed development, and the famous octuple 2-pdr Pom-Pom didn't enter service for another decade.² It was a revolutionary weapon, introducing director control³ and power operation to automatic AA weapons. The 2-pound shell was powerful enough for aircraft of the era, with a 0.16 lb bursting charge. On the downside, rate of fire was only 90 rds/min/gun, and the choice to use existing ammunition limited muzzle velocity to only 1,900 ft/sec. By the late 30s, this was clearly insufficient, and an improved version capable of about 2,300 ft/sec was designed. It was not capable of firing the old ammo, and for some bizarre reason, both low and high-velocity weapons were kept in production throughout the war.

Unrotated Projectile launchers atop the turrets of HMS Nelson

Unfortunately, the 2-pdr was clearly on its last legs by the beginning of the war. Even the high-velocity version was obsolescent by the end of hostilities, and the small shell was also not very effective against the much more durable aircraft that were filling the skies. But they sprouted from British battleships in great numbers. The WWI-era battleships had 2-4 octuple mounts each, while the King George V class ships were commissioned with four mounts, and ended the war with as many as eight octuples and six quads. Many of these ships also carried what was known as the Unrotated Projectile, a cover name for a rocket that would deploy a cable with an explosive charge attached. The idea was to entangle the attacking aircraft and destroy it, but in practice these "aerial mines" proved easy to avoid and the mounts were replaced by late 1941 with more conventional AA weapons.

1.1" quad mount aboard Pennsylvania.

On the other side of the Atlantic, the USN was somewhat slower to respond to the threat. It relied solely on machine guns until the late 20s, when development began on a 1.1" weapon firing a shell of about 1 lb. These weapons, in quad mounts, were entering the fleet at the start of the war, with the treaty battleships designed to carry four of them. One unusual feature of this mount was that the barrels pivoted side-to-side to allow them to track dive bombers at high angles, when a small change in angle might require a large change in train. However, the mount was unreliable in service and the lightweight shell was grossly insufficient for the task at hand. Something better would be needed, and it came, oddly enough, from neutral Sweden.

40mm guns in action aboard New Jersey. Note the 4-round clips being passed to the guns.

The 40mm Bofors was undoubtedly the most successful automatic AA gun of the war, used by every major naval power in one form or another. It fired a 2-lb shell at about 2,800 ft/sec, and could maintain a sustained rate of 90 rpm per barrel thanks to the use of four-round clips which were loaded as the gun fired.⁴ The original design was complex and difficult to build,⁵ but the US redesigned the weapon for proper mass production, and Chrysler and its subcontractors built 60,000 guns over the course of the war, at half the projected unit cost. Many of these weapons found their way aboard ships, in single, twin, and quad mounts. These, like the 2-pdr, were power-operated and driven by directors, and they took a ferocious toll on Axis aircraft. In the Pacific, the Bofors was responsible for a full third of Japanese aircraft shot down by American AA gunfire, more than any other weapon. The British also made use of the Bofors, although it served alongside the 2-pdr throughout the war. After the war, both navies kept it in service, and a few weapons remain in use to this day.

Quad 40mm Bofors mount onboard Massachusetts

But even these weapons had their drawbacks. They were large and heavy, the quad 40 weighing as much as a single 5"/38, and were not quite adequate at very close range. A particular problem was that they, like most naval guns, had separate pointers and trainers, which reduced their ability to respond to targets close in. Initially, the Americans and British both used .50 cal machine guns, but these were seen as inadequate for the task even before war broke out. Weirdly, the solution to this problem came from another neutral country, Switzerland.⁶

20mm Oerlikon aboard Iowa

The 20mm Oerlikon was usually mounted as a free-swinging weapon, controlled by a single gunner looking through a ring sight.⁷ While the bullets weighed only an eighth of what those of the Bofors did, it could empty its 60-round drum magazine in only 8 seconds, and needed only to be bolted to the deck to be used. The ability to keep fighting after damage that took out the ship's power was a major advantage, as shown most graphically when Repulse and Prince of Wales were sunk. The 20mm was very effective early in the war, getting almost half of AA kills on Japanese aircraft in the second half of 1942.⁸ Against the Kamikazes, however, it was reported that the main value of the 20mm battery was to warn the rest of the crew that a plane was about to hit.

20mm Oerlikon aboard Cassin Young, with Mk 14 sight⁹

So did America contribute nothing to the field of light AA beyond mass production? Certainly not. At the beginning of the war, the typical light AA weapon was aimed by use of a ring sight, a simple device which gave the gunner some idea where to start with his aiming. Tracers would then hopefully let him walk his fire onto the target. A team at MIT under Charles Stark Draper was charged with developing something better.¹⁰ The result was the Mk 14 gunsight. This device used a pair of gyros to measure the rate of rotation of the sight as the gunner tracked the target. This rate was used to displace mirrors which gave the correct lead for the target, at least at close range.¹¹ A second man was used to set the range on the sight. In addition to being mounted directly on the 20mm Oerlikon, the Mk 14 was fitted to a "dummy gun" and became the Mk 51 director, which was widely used for 1.1" and 40mm guns, and even linked to the 5" guns towards the end of the war, to attack surprise targets the Mk 37 couldn't track effectively.¹²

German 3.7cm SK C/30 in twin stabilized mount

Both Axis powers made at least some use of the Bofors, but in neither was it the mainstay of their air defenses. The Japanese used a triple 25mm that was inferior to the 40mm in range, lethality, and rate of fire. For some reason, they considered it an excellent weapon.¹³ The Germans had a 37mm gun with reasonable performance, apart from one major flaw. It predated the end of the Treaty of Versailles, when Germany was not allowed heavy automatic weapons, and was thus semi-automatic and could only fire about 30 rpm. The Germans thus relied more on their own 20mm design, but it suffered the same drawbacks as the Oerlikon.

Mk 51 director aboard Massachusetts

The war saw dramatic increases not only in the capability of light AA weapons, but also in their number. Iowa, originally designed with only four quad 1.1" and a few .50 cal machine guns, was completed with 15 quad 40s, a count which grew to 19 before she sailed for the Pacific,¹⁴ and 52 single and 8 twin Oerlikons. In this, American ships, larger and with greater reserves of stability, had a major advantage over their British counterparts. That said, it was not uncommon for destroyers to lose main guns and torpedo tubes to provide space and weight for light weapons. Even aboard the battleships, the addition of the guns was not without problems. Their crews crowded the ships with almost 50% more men than they were originally designed for, and weapons had to be placed in locations that complicated ammunition supply, suffered damage in rough seas, or were completely untenable due to muzzle blast if the main guns were in action. More guns were placed amidships, replacing the boats that had formerly occupied the space. But by 1945, the American battleships were some of the most formidable anti-air platforms afloat, although the extra mounts disappeared quickly after the war, to save on manpower and the cost of keeping them operational.

While light AA weapons were always something of an afterthought to the designers of battleships, they were vital to their performance during WWII. Ultimately, the entire category was rendered obsolete by better aircraft and the first guided weapons, but for a few years, they achieved a prominence equivalent to the other batteries.