The AC-130 30mm 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 the most common round used in the Mk44. From higher altitudes (up to 20,000 feet), the AC-130 fire control system and night vision sensors, enable the 30mm gunners to accurately hit targets with high explosive shells.
The existing 25mm and 40mm guns are being phased out of military service, and the new 30mm gun is easier to operate. All 25 of the AC-130s are being converted to use the 30mm guns.
While the A-10 still gets a lot of work in Iraq and Afghanistan, there are still large stocks of 30mm ammo for it, that will never be fired given the current use rates. So it makes sense to repackage it for use on the new 30mm cannon on the AC-103s. The same ammo is being used in Mk46 30mm guns for the new U.S. Marine Corps amphibious vehicle (the AAAV) and for San Antonio-class (LPD 17) amphibious ships.
The U.S. Air Force is spending nearly $10 million to repackage several hundred thousand rounds of 30mm cannon ammo used in it's A-10 aircraft, for use in AC-130 gunships. Last year, the air force began replacing the 25mm and 40mm guns on its AC-130 gunships, with two 30mm cannon. The Mk44 30mm Bushmaster weighs 344 pounds and fires at a rate of 200 or 400 rounds per minute (up to 7 per second). While the 30mm guns used on the A-10 and AC-130 are quite different, they use the same size ammo.