Jun 20

A Gun to Counter the Dive Bomber

Monday, June 20, 2016 12:01 AM


A gun crew practices on a quadruple 1.1-inch mount at Dam Neck Training Center Virginia. Note the large, cumbersome magazines. (National Archives)

A gun crew practices on a quadruple 1.1-inch mount at Dam Neck Training Center Virginia. Note the large, cumbersome magazines. (National Archives)

The quadruple 1.1-inch machine cannon, affectionately known as the “Chicago Piano,” was the first medium-range antiaircraft gun adopted by the U.S. Navy.1 Engineered and built by the Naval Gun Factory during the Great Depression, it was designed specifically to combat dive bombers. The four-barreled weapon fired a one-pound explosive shell that was fused to explode on contact with the thin doped fabric that covered the wings of the era’s biplanes. The resulting shrapnel would tear through the wing, causing loss of control.

The need to provide the Fleet with a new antiaircraft gun became evident in the late 1920s in response to the growing threat posed by dive-bombing. By the beginning of 1929 a number of senior officers were becoming increasingly concerned about this new form of attack. The problem of defending against the dive bomber was articulated by the director of Fleet Training, Rear Admiral Luke McNamee. “Even if the defensive fire begins at the earliest possible instant,” he wrote in a memorandum addressed to the chief of Naval Operations, “. . . only about 30 seconds of actual firing times are available to get on the attacking planes and damage them sufficiently to prevent the attack from being completed.” None of the antiaircraft guns then available—the .50-caliber machine gun, 3-inch antiaircraft gun, and 5-inch/25-caliber dual-purpose gun—was designed for this purpose.

One proposal put forth by the Bureau of Ordnance (BuOrd), which had been studying various weapons to counter the dive bomber, called for a multiple-barrel 1.1-inch cannon firing an explosive projectile fitted with a supersensitive fuse that would detonate on contact. Admiral McNamee recommended that the development of this weapon be pushed and “a gun of this type be perfected and issued to the service as may be practicable.”

Although BuOrd recognized the urgent need for a new weapon to combat dive bombers, opinion varied within the bureau as to which caliber of ammunition would be the most effective. Thus, the bureau began conducting head-to-head firing tests using .30-caliber, .50-caliber, and 1.1-inch ammunition against statically mounted obsolete aircraft at the Naval Proving Ground in Dahlgren, Virginia. The results showed that while .30-caliber bullets were capable of disabling an engine, they would be ineffective in preventing a dive bomber from pushing home its attack once it had begun. The .50-caliber made an excellent showing—in many cases its heavy projectiles penetrated through the engine block to the pilot’s position—but the impact of a .50-caliber bullet was not likely to cause loss of control of the airplane unless the pilot was injured.

Total loss of control was the only damage worth considering, as it was the only hit that would prevent a dive bomber from completing its attack. Thus the bureau favored the 1.1-inch gun and its explosive projectile, which was likely to cause loss of control if it struck a plane’s wing. The 1.1-inch gun’s volume of fire, however, was considerably less than that of the .50-caliber; to have the same volume, the 1.1-inch would need to be installed in a quadruple mount. Such a mount would be much heavier than the .50-caliber’s, would take up more deck space, and would be considerably more expensive.

In the final analysis, ordnance specialists within BuOrd concluded that because of the small target size of a dive bomber, providing “a weapon of greater destructiveness than [could] be delivered by a solid [.50-caliber] projectile” was essential. Their recommendation convinced the chief of the bureau, Rear Admiral William D. Leahy, to proceed with the development of the 1.1-inch machine cannon and its quadruple mount.

Work on the new gun progressed slowly because of Great Depression economic restrictions placed on the Navy. Tests on the firing range at Dahlgren did not begin until March 1931. It took another three years to iron out all of the engineering difficulties encountered in designing the power-operated mount, a complicated piece of machinery that possessed two unusual characteristics to counter the dive-bomber threat. The mount elevated the gun barrels past the normal 90 degrees and had a traversing mechanism in the plane of the four barrels that allowed the gun’s aim to be adjusted at very high angles of fire.


Ammunition was fed into each of the four water-cooled guns from seven-shot magazines inserted into cradles above each gun. The cradles held two magazines: one in firing position and the other in the standby position. As the first magazine was emptied it was automatically released, allowing the second magazine to drop into place. Empty cartridge cases were ejected from a breech housing under the magazine.

Firing tests of the first quadruple 1.1-inch gun mounts delivered to the Fleet were conducted by the destroyers Phelps (DD-360) and Clark (DD-361) in the spring of 1937 and revealed several unexpected problems. Gunners on board both ships complained of vibration and smoke that hindered their ability to sight the target. The Clark’s gun crews “had not mastered the proper adjustment of the recoil spring,” which caused the gun to jam, a problem that would plague the weapon throughout its service life. The gun crews in the Phelps were able to overcome this “stoppage problem.”


Over the course of the next year, the target ship Utah (AG-16), which received one of the earliest installations of the quadruple 1.1-inch gun, conducted an intensive effort to develop a practical and effective means of fire control that would overcome the sighting problem. Vibration and smoke continued to be problems, interfering with fire control and causing a serious hindrance to spotting. The vibration was so severe, wrote the Utah’s commanding officer, that “Beginning with the first salvo, gunlayers are jostled in their seats, receiving continued body shocks. At the same time sights begin vibrating wildly. This in conjunction with smoke and blast, constitutes a major difficulty to sighting.”

To overcome this problem BuOrd, with help from the Utah, designed a remote-control director—the Mark 44—for the quadruple 1.1-inch gun that would provide adjustments in train and elevations.2 Unfortunately, deliveries of the director did not begin until the last few months of 1941 so that most of the Navy’s ships—including the Lexington (CV-2) and Enterprise (CV-6)—entered the war without this essential piece of equipment.

In service, gun crews found the 1.1-inch gun difficult to maintain and prone to failure. The gun and mount suffered from a number of problems, including a tendency to overheat and jam after a few rounds as the weapon warmed up. It worked OK if properly maintained, but maintenance was a nightmare, especially for new recruits. One source noted that the weapon required the services of a first-rate gunner’s mate just to keep it firing. Walter Becker, who served in the Clark, recalled that during the Lexington’s abortive February 1942 raid on Rabaul one gun captain tried to keep his 1.1-inch going with a wooden mallet and a liberal use of profanity.

The gun’s ammunition was also problematic. The fuse on the high-explosive projectile contained a short time-delay that theoretically allowed the projectile to penetrate an aircraft’s skin before detonating. But the highly sensitive fuse, which armed upon firing, had no safety device. Premature explosions within the gun barrel itself during gunnery practice were not uncommon. Needless to say, this caused great discomfort to both crew and bystanders. According to one account, the antiaircraft gunnery officer of the heavy cruiser San Francisco (CA-38) refused to conduct live-fire training because it put the lives of the gunners in jeopardy.

While 1.1-inch guns rendered valuable service in early Pacific battles, 40-mm Bofors replaced them in most Navy warships. Destroyer escorts, which were primarily used to hunt down submarines--not guard against air attack--continued to make due with 1.1 guns, including these on board the USS FRANCIS M. ROBINSON (DE-220). (National Archives)

While 1.1-inch guns rendered valuable service in early Pacific battles, 40-mm Bofors replaced them in most Navy warships. Destroyer escorts, primarily used to hunt down submarines–not guard against air attack–continued to make due with 1.1 guns, including these on board the USS FRANCIS M. ROBINSON (DE-220). (National Archives)

Regardless of these problems, the 1.1-inch guns were a welcome addition to the Fleet’s antiaircraft defenses and played an important role in protecting U.S. carriers at the battles of the Coral Sea and Midway.3 Nevertheless, by the time the gun was installed in meaningful numbers, advances in aircraft construction, such as aluminum replacing doped fabric, had rendered the 1.1-inch’s ammunition less effective than initially conceived. That, combined with difficulties in providing a lead-computing director and problems with its powered mount, led to the weapon’s replacement by the 40-mm Bofors/Mark 51 director combination in every ship class except destroyer escorts. credit-n.ru