The USAF Is Rebuilding World War II-Era 40mm Shells for its AC-130U Gunships

The service is the last user of the Bofors cannon in the US military and has had to go hunting for more ammunition.

Clemens Vasters via Wikimedia

The U.S. Air Force is rebuilding tens of thousands of World War II-era cannon rounds specifically for the 40mm cannons on its AC-130U Spooky II. Though the service has long been looking to finally retire this particular part of the gunship’s arsenal, which has increasingly become a logistical nightmare, the aging guns have so far proven too effective to get rid of completely.

In November 2017, the U.S. Air Force revealed that the 780th Test Systems Squadron at Eglin Air Force Base in Florida had been testing the upgraded 40mm high explosive ammunition, known as the PGU-9D/B, earlier in the year. The new version combines decades old components with a fuze that is safer and more reliable than the original model. Some of the brass cartridge cases had production date stamps dating to 1944.

In all, the service plans to rebuild approximately 80,000 older rounds into the new configuration. The 780th developed the process to modify the existing ammunition and build a number of prototype rounds, but it is unclear whether the unit, which handles various munitions testing duties, will perform the rest of the work or how long the process might take in total. Air Force Special Operations Command (AFSOC) controls all of the service’s AC-130 gunships, which include the Us, as well as newer AC-130W Stinger IIs and still in development AC-130J Ghostriders.

The project does speak to the continued importance of both the AC-130U and its 40mm Bofors cannon. To give a sense of the demand for the aircraft, according to an official Air Force history, seven of the Spooky IIs assigned to the 4th Special Operations Squadron flew nearly 4,000 combat hours between November 2013 and June 2014. The planes and their crews spent more than 1,175 days deployed to conflict zones during that time.

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Members of the 780th Test Systems Squadron load a PGU-9D/B round into a test gun during testing.

In October 2017, members of the crew of one of the 4th's AC-130Us, which had gone by the callsign Spooky 43, received Distinguished Flying Crosses and other medals for valor for one particularly intense mission over Afghanistan in 2016. The gunship beat back insurgents who had ambushed a team of U.S. Army Special Forces personnel from three sides, having to fire both 40mm shells and rounds from the aircraft's larger 105mm howitzer dangerously close to friendly troops.

At present, the Spooky II is the only system of any kind left in the U.S. military to use the weapon in any capacity. In addition to the PGU-9 rounds, gunners can fire high explosive-incendiary, armor-piercing shells, or a mixture of all three types, depending on the target set.

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An AC-130U Spooky II gunship.

The United States first adopted the Swedish-designed guns during World War II for anti-aircraft use at sea and on land. Chrysler built some 60,000 of them in the United States under license. The last of these air defense weapons were out of service at least by the end of the 1970s, if not earlier.  

However, in 1969, the Air Force had begun to acquire some of the weapons for an entirely different purpose. At that time, its AC-130A and AC-119K gunships were flying up and down the Ho Chi Minh Trail in Laos attempting to stem the flow of personnel, weapons, and other supplies from North Vietnam into South Vietnam.

Despite the trail moniker, the North Vietnam military had turned many sections into a functional highway. Dense anti-aircraft defenses, including 37mm and 57mm guns, defended the route, quickly became a significant threat to the Air Force’s low and slow flying modified cargo aircraft.

With a pair of 40mm cannons in place of two of the four 20mm Vulcan cannons on earlier versions, AC-130 crews could fly higher and further from enemy air defenses and still accurately hit trucks and personnel below. The new armament turned out to be so successful that the Air Force refitted the earlier A model gunships with more powerful guns and converted more powerful C-130Es to the same standard.

The Air Force eventually replaced one of the 40mm Bofors with an even larger 105mm howitzer on later E versions, an armament configuration that became a feature on later AC-130H and AC-130U gunships. The service retired the last of the H variants in 2015, leaving just the U types with the older guns, which continue to fly combat missions in war zones such as Afghanistan.

In 2007, the Air Force did experiment with replacing the Spooky II’s 40mm gun, as well as its 25mm GAU-12/U rotary barrel cannon, with a pair of 30mm weapons. Those tests did not produce the desired results, with the service complaining about a lack of accuracy, the aircraft returned to their original configuration the following year.

The 30mm gun has become a standard weapon on the newer AC-130W and will be part of the AC-130J’s armament package, too. As we at The War Zone reported in October 2017, there have continued to be issues with the accuracy and reliability of the weapon’s ammunition, though.

At the same time, after more than seven decades of steady service and nearly 50 years as a gunship weapon, the Bofors cannons have only become more difficult to maintain. The Swedish firm, now a division of the U.K.-headquartered defense contractor BAE Systems, still builds 40mm guns, but the its more recent versions does not use the same ammunition as the older American variants.

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The bottom of one of the new PGU-9D/B rounds, showing the 1944 date stamp.

The older rounds are increasingly hard to come by, as evidenced by the Air Force’s project to rebuild existing surplus, but the issues go beyond just finding more ammunition to feed the Air Force’s needs. The cannon barrels, among other components, do not last forever, with heavy use wearing down the rifling and their basic structure, making them increasingly less accurate and even dangerous to use. By 2013, the Air Force was essentially having to custom order new barrels at a price of $1.3 million each.

The year before, the Air Force Special Operations Command (AFSOC) sent a team Greece to see about buying up a stockpile of old barrels that the NATO member had in storage after it retired the last of its versions in 2005. In the end, the service managed to pick up nearly 140 barrels and other rare parts on the cheap to keep its own weapons in working order, saving a total of $14 million in the process.

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Bill Walter, right, inspecting a 1950s production 40mm barrel still in its original packing crate in Greece in 2012. 

“I spent two days combing through several arsenals searching for 40 mm parts,” Bill Walter, then a program analyst at AFSOC’s Strike Requirements Branch, said at the time. “It was just like going into a museum. I just felt awestruck. It was like I stepped into another time, like stepping back 50 or 60 years.”

This increasingly complicated situation may not be an issue for much longer. In 2015, the Air Force retired the first AC-130U. The service expects to continue sending members of the fleet to the boneyard through 2018 as it gets closer to achieving an initial operating capability with the new AC-130J.

When the last Spooky II leaves service, it will mark the end of both the aircraft’s own impressive career and the final retirement of the Bofors cannon after what will be at least 75 years in service with the U.S. military.  

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