Guns

Photograph of 14"/45 guns on USS Pennsylvania

Naval guns remained an important category of weapon in spite of the ascendancy of aircraft. They served in three main roles.

Antiship. The heaviest guns on a naval vessel were usually intended for the antiship role. Battleship guns could hurl shells that were 12" to 16" (30 to 40 cm) in diameter and weighed a good fraction of a ton to distances of up to 30 miles (56 km). Cruiser guns were much smaller in caliber (6" or 8" or 15 to 20 cm), still comparable with the heaviest Army artillery. Destroyer guns were comparable to divisional artillery at 4" to 5" (10 to 13 cm), while minor warships typically carried 3" (76mm) main armament.

Antiship rounds were classified as armor piercing or high explosive. Armor piercing shells were made of hardened steel, sometimes with a softer nose cap, and carried a small bursting charge with a delayed fuse. These shells were designed to penetrate armor plate through sheer brute force, with the fuse setting off the bursting charge twenty or thirty milliseconds later. This gave the shell time to pass into the interior of the ship where its fragments would do the most damage. High explosive shells were intended for use against unarmored targets and usually exploded on impact. Whereas an armor piercing shell carried perhaps 5% of its weight as high explosive, a high explosive shell would carry about 20% of its weight as high explosive.

Shore bombardment. Naval guns supplied heavy firepower for a landing force while it was unloading its own artillery. Naval guns were also occasionally turned against shore installations during raids against enemy coasts. Most admirals of the Second World War were hesitant to order their ships engage in a duel with coastal artillery ("A ship's a fool to fight a fort"), but the Americans became increasingly proficient at destroying fortifications on landing beaches, bringing their ships in very close to shore to improve accuracy. Even so, shore bombardments often proved inadequate to protect the landings, as at Iwo Jima.

Antiaircraft. As the war progressed, both the Allied and Japanese navies mounted every-increasing numbers of antiaircraft guns on their naval vessels.

Gun accuracy was only fair even under the best of conditions. Morison (1950) reports that, during the night action at Empress Augusta Bay, the American force fired 4591 6" shells and scored perhaps 20 hits. This hit probability of 0.4% was not unusual for the Solomons campaign, where most of the surface actions of the war were fought.

Some of the important characteristics of guns are:

Ammunition type. Naval ammunition is classified as either fixed, semi-fixed, or bagged. Fixed ammunition resembles cartridges for small arms, with the projectile permanently clamped to a brass case containing the propellant charge and primer. This was the rule for ammunition for guns smaller than 6". Semi-fixed ammunition had a propellant charge in a brass case containing the primer that was separate from the projectile, and it was typically used in 5" to 8" guns. Bagged ammunition, typical of the largest naval guns, consisted of separate projectiles, silk bags of propellant, and primers. In principle, gun crews could reduce the muzzle velocity and thus the range of a large naval gun by using fewer bags of propellant. This would not be desirable when firing armor piercing shells, but it might be advantageous for high explosive shells, reducing barrel wear and allowing plunging fire on shore installations.

Semi-fixed ammunition has some advantages even at very large calibers and was used for even the largest guns by the German Navy. The powder cartridges are safer, being better protected against penetrating hits to the ship's magazines and having less potential for flareback accidents when the gun breech is opened for a new round. Cartridges also help to seal the breech, allowing simpler breech designs with the potential for higher rates of fire. However, this is partially negated by the heavier and more complex handling equipment required for cartridges. All the major participants in the Pacific War used bagged ammunition for their large-caliber guns.

The projectile itself might be one of several types. Armor piercing (AP) and high explosive (HE) shells have already been mentioned, but in addition a gun might be supplied with timed or proximity fused shells for antiaircraft use. Shells for automatic guns could carry a tracer to allow the gunners to better judge the trajectory of the shells, and the smallest caliber guns were sometimes supplied with incendiary shells or solid shot. Star shells were designed, not to inflict damage directly, but to illuminate enemy ships or positions at night.

The Japanese came up with an interesting variant of the armor piercing shell, which was called the Type 91 AP Shell or diving shell. This shell was designed such that if it fell slightly short, it would continue through the water to hit the hull of the target ship well below the waterline, where the armor was light or nonexistent. This had the potential to cause extensive flooding. The Type 91 had a blunt nose with a thin aerodynamic cap that shattered on entering the water. American ordnance experts deduced the properties of the Type 91 from examining the damage from its one success, the near-destruction of Boise. They noted that it had poor penetration against heavier armor, and no attempt was made to duplicate it.

Projectile weight. The kinetic energy of a projectile is proportional to its mass times the square of its velocity. The weight of a high explosive shell is also a rough measure of its explosive power. Ceteris paribus, a heavier projectile is more destructive. A lighter projectile generally loses its punch more quickly at longer ranges than a heavier projectile.

Velocity. This is the muzzle velocity of the projectile fired by the gun. A faster projectile is much more destructive, since its kinetic energy is proportional to its mass times the square of its velocity. Faster projectiles also have a flatter trajectory, which is sometimes a disadvantage.

Maximum elevation. This is the maximum firing angle of the gun as typically mounted. It affects the usefulness of the gun against aircraft, and it also determines whether the gun can use plunging fire against distant targets. Plunging fire is fire that comes in at a steep angle, so that the shells strike the deck of an enemy ship rather than the sides. In general, armored ships of the Second World War had lighter horizontal than vertical armor. Plunging fire was also more effective against land fortifications than flat trajectory fire, for similar reasons. The older U.S. battleships were particularly useful for delivering plunging fire during shore bombardment.

Range. The maximum horizontal distance to which the gun can fire a projectile.

Altitude. For antiaircraft guns, this determines the maximum altitude its projectiles can reach.

Rate of fire. Nominal rate of fire of the gun. This varied from about two rounds a minute for battleship guns to hundreds of rounds per minute for the lightest antiaircraft weapons. The firing cycle (time to load and fire a single round) is sometimes specified instead, especially for the heavier guns. In general, the heavier the gun, the lower the rate of fire. Guns between 3" and 6" diameter were sometimes described as quick-firing guns since they could fire several rounds a minute, important when fending off destroyer attack (their original role.) Guns smaller than 3" were usually automatic weapons.

Loadout. The typical number of rounds available for the gun when the magazines are fully loaded. This could vary considerably for the same gun on different classes of ships and at different times. For example, the "standard" allotment for 5" guns on U.S. destroyers prior to war was 100 rounds per gun, but the "mobilization" allotment was typically 300 rounds per gun. This proved inadequate, and by the time the war ended, the allotment was approaching 600 rounds per gun.

References

Campbell (1985)

Frank (1990)
Garzke and Dullin (1985)

Morison (1950)

Okun (2003)

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