What guns are used on the ac 130?

The AC-130s that are flying today are either AC-130H "Spectre" or AC-130U "Spooky" gunships. Both variants are armed with a 40mm Bofors cannon and a 105mm howitzer. The AC-130H originally also fitted a pair of 20mm Vulcan gatling guns but those have been removed. The AC-130U mounts a 25mm gatling gun.

But seeing as how it's made out of a C-130, you could stick just about anything you wanted back there.