The C130 is a cargo aircraft. The AC130 is the airframe converted for use as a gunship, of which there are several variants.
Guns under an aircraft are usually found on helicopters, mounted on turrets. Purpose built aircraft usually integrate guns into the wings (fighters) or fuselage (A10). WW2 bombers like the Flying Fortress worked more like the AC130, and had gunners in various places including a belly mounted turret.
The low load floor of a cargo aircraft precludes placement of guns there. Additionally, the AC130 carries multiple guns. 7.62 Minigun, 25mm Gatling gun, 40mm Bofors cannon, and 105mm howitzer have all been used. Simultaneous use requires multiple mounting points. Same side placement means they can all be used on one target.
105mm Howitzer in operation.
