The U.S. Air Force has put in a rush order for new 30mm ammo for its AC-130 gunships. This came about because recent fighting in Iraq and Syria showed that the existing PGU-13D/B 30mm rounds were less accurate than expected at max range and also had a high dud rate. The PG-46/B 30mm round was designed specifically for the AC-130 30mm cannon and is more accurate at long range and has fewer duds. SOCOM (Special Operations Command) had people on the ground (as well as operating the AC-130s) while this was going on so the air force was acting on solid evidence that there was a problem.
This sort of thing had not been a problem before but since 2010 two Mk44 30mm cannon have replaced the 25mm and 40mm cannon on air force AC-130 gunships. Testing had shown that at longer ranges the 40mm autocannon shells would be accurate and reliable while the 30mm round was less accurate but it was believed this would not be a problem in combat. Nevertheless an improved PG-46/B 30mm round was also available and it handled the accuracy and dud rate problems. But there was still a lot of the older PGU-13D/B rounds available and these were the ones that caused problems in Iraq and Syria.
The large quantities of PGU-13D/B ammo available was because the Mk44 30mm Bushmaster autocannon used in the AC-130 and the 30mm autocannon used on the A-10 aircraft are quite different, but they use the same size ammo, and a lot of A-10 30mm ammo was repackaged as PGU-13D/B for use on AC-130s. There were large stocks of 30mm A-10 ammo, and the AC-130 used more of it than the A-10 did. The PGU-13D/B worked well enough initially at shorter ranges and in situations where the dud rate was not critical. But as crews and troops on the ground got more accustomed to the 30mm autocannon and began to depend on it as a last ditch weapon for supporting troops in close proximity to the enemy (and nearby civilians) the accuracy and dud rate became an issue. This was especially the case when the Bushmaster was firing at extreme ranges.
The 30mm Bushmaster weighs 156 kg (344 pounds) and fires at a rate of 200 rounds per minute (up to 4 per second). It has 160 rounds available before needing a reload. That means the gunner has 40-50 seconds worth of ammo. Each 30mm round weighs about 715 g (25 ounces) depending on type. Explosive anti-personnel rounds are the most common round used in the Mk44. From higher altitudes (up to 6,100 meters/20,000 feet), the AC-130 fire control system and night vision sensors, enable the 30mm gunners to accurately hit targets with the high explosive shells. The 30mm round is widely used in ground weapons as well and new 30mm rounds are usually developed for those weapons.
The existing 25mm and 40mm guns on the AC-130 are being phased out of military service. Actually, the 40mm gun is something of a museum piece. None have been manufactured since the 1990s and parts have to be custom made. Ammo is hard to get, and expensive, as well. The new 30mm Bushmaster is easier to operate and maintain. For example, many repairs can be made while in the air, and it's easier to reload.
All of the older AC-130s were converted to use the 30mm guns and the new AC-130J model has it as standard. At this point all AC-130s, are armed with two 30mm cannon and a 105mm howitzer, rather than a 40mm and a 25mm cannon and a 105mm howitzer.