July 17, 2012:
The U.S. Air Force has officially accepted the modified 30mm Mk44 Bushmaster automatic cannon as the GAU-23. For the last three years, modified (and continually tweaked) Mk44s have been operating on a dozen U.S. Air Force AC-130 gunships and more recently as part of the U.S. Marine Corps Harvest Hawk ("instant gunship" via several pallets of sensors and weapons) version of the KC-130J tanker.
The 30mm Bushmaster cannon weighs 157 kg (344 pounds) and fires at 200 or 400 rounds per minute (up to 7 per second). The Bushmaster 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 high explosive/incendiary round weighs about 714 g (25 ounces, depending on type). The fire control system and night vision sensors enables the 30mm gunners to accurately hit targets with high explosive shells. Earlier SOCOM AC-130 gunships are armed with a 105mm howitzer, a 25mm and 40mm automatic cannon. But the two smaller caliber guns are being phased out of military service. The air force is now equipping its gunships just with smart bombs and missiles, as well as one or two GAU-23s.
The big thing with gunships is their sensors, not their weapons. Operating at night, the gunships can see what is going on below, in great detail. Using onboard weapons, gunships can immediately engage targets. But with the appearance of smart bombs (GPS and laser guided), aerial weapons are now capable of taking out just about any target. So gunships can hit targets that were "time sensitive" (had to be hit before they got away) but could also call on smart bombs or laser guided missiles for targets that weren't going anywhere right away. Most of what gunships do in Afghanistan is look for roadside bombs or the guys who plant them. These gunships want to track back to their base and then take out an entire roadside bomb operation.