May 17, 2017:
As a result of combat experience U.S. SOCOM (Special Operations Command) has reversed a 2005 decision to stop using the 105mm cannon on its gunships. That means the 32 new AC-130J Ghostrider gunships underwent a final revision that reinstated the 105mm cannon. In early 2017 this new 105mm installation completed testing. The first AC-130Js entered service in 2016. The 32 new AC-130Js will replace the current force of 28 older model gunships (AC-130Hs, AC-130Ws and AC-130Us) by 2021. In the meantime the few older AC-130s that had their 105mm gun removed are getting it back.
Since 2010 the design of the AC-130J has gone through several changes about what weapons worked best. In 2005 it was decided that all AC-130s would just be equipped with guided missiles and smart bombs. That meant eliminating the long used 40mm and 20mm autocannon and 105mm cannon. But it soon became apparent that that cannon were still needed in many situations. Thus SOCOM first decided to bring back autocannon and install one 30mm cannon (to replace the rather elderly 40mm and 20mm models). The AC-130 crews and the troops on the ground also made a strong case for retaining the 105mm cannon, now fired out the back of the aircraft via a modified rear ramp.
Meanwhile SOCOM has standardized on the Griffin missile launched from inside the aircraft and eight laser guided Hellfire missiles or GPS guided SDB (small diameter bomb) suspended from the wings. These guided weapons had proved themselves in combat and allowed AC-130s to regularly operate in daylight. When using the SDB and missiles the AC-130J can fly high enough to stay out of range of ground fire. But with the cannon the gunship must fly much lower, where the sensors, and all weapons, are more effective if only because the missiles and bombs arrive on target more quickly and the 30mm and 105mm cannon can add their firepower. When using the cannon the AC-130J only operates at night.
The AGM-176 Griffin weighs 15 Kg (33 pounds, or 20.5 kg/45 pounds with the launch tube) and has a 5.9 kg (13 pound) warhead. Griffin was first used in Afghanistan. Griffin has a greater range (20 kilometers from aircraft for the B and C versions) than Hellfire because of pop-out wings that allow it to glide after launch. The latest version (Griffin C) uses laser, GPS, inertial guidance and two way communications.
The 129 kg (285 pound) Small Diameter Bomb (SDB) is a GPS guided smart bomb designed for urban warfare and targets very close to friendly troops. The smaller blast (17kg/38 pounds of explosives, compared to 127 kg/280 pounds for the 500 pound bomb) from an SDB results in fewer civilian casualties. Friendly troops can be closer to the target when an SDB explodes. Griffin and SDB hang from the wings of the AC-130J.
The Hellfire II weighs 48.2 kg (106 pounds), carries a 9 kg (20 pound) warhead, and has a range of 8,000 meters. Hellfire has been popular because it is easy to use, reliable and very effective. For what it can do, Hellfire is very cost-effective. But not so much as the 105mm cannon, whose shells cost $400 each.
The Mk44 30mm Bushmaster cannon weighs 157 kg (344 pounds) and fires at 200 or 400 rounds per minute (up to 7 per second). The cannon has 160 rounds available before needing a reload. That means the gunner has 25-50 seconds worth of ammo, depending on rate of fire used. Each 30mm round weighs about 714 g (25 ounces, depending on type). Explosive anti-personnel rounds are fired when used in gunships. The fire control system, and night vision sensors, enable the 30mm gunners to accurately hit targets with high explosive shells.
The 105mm cannon on AC-130s is a modified (to weigh about 1.4 tons) version of the M102 howitzer that was used by light infantry units from the 1960s to the 1990s. The M102 fires a 15 kg (33 pound) shell. The complete round (with casing and propellant) weighs about 19 kg (42 pounds). On the ground the 105mm fires at distant targets it cannot see, with the shell following a curved trajectory to hit something up to 11 kilometers away. On the gunship it fires directly at targets the gunship sensors can see and that shortens the range to about 1,100 meters. On the gunship the 105mm can fire up one round every ten seconds. Usually only one round per target is needed. In the older AC-130s 96 105mm rounds were carried. The larger AC-130J can carry twice as many, if not more.