The existing 25mm and 40mm guns are being phased out of military service, and the new 30mm gun is easier to operate. The first four AC-130s converted to use the 30mm guns, will be available later this year, with the rest of the 21 AC-130s converted next year.
SOCOM (U.S. Special Operations Command) is replacing the 25mm and 40mm guns on its AC-130 gunships, with two 30mm Bushmaster cannon. The Mk44 30mm Bushmaster cannon weighs 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 25 ounces (depending on type.) The anti-armor shell weighs about half a pound. The armor piercing round will go through 25mm of steel at 2,000 meters range. This will get through the top armor of most vehicles, and spray the inside with fragments. At that range, time of flight is about 1.7 seconds. Explosive anti-personnel rounds are also available. From higher altitudes (up to 6,000 meters), the AC-130 fire control system and night vision sensors, enable the 30mm gunners to accurately hit targets with high explosive shells.