The GAU-12 25mm six-barrel gun was the "cannon" armament used during some attempted UFO interceptions (due to the interceptors being an American design). The cannon was mounted internally, but had limited ammunition, range and accuracy. Only four interceptions were attempted using the GAU-12; of those, one resulted in the loss of the interceptor, two resulted in a failed interception, and one was a limited success. It was soon determined that interception using cannon was almost impossible due to the durability, speed, and maneuverability of alien UFOs. The one success was a small scout. Against any larger UFO (and they're all larger than a small scout), engaging with cannon was considered too dangerous and ineffective.
The GAU-12 has a 300 round capacity with a lead computing optical sight system (LCOSS) gunsight. The Marines use a 25mm depleted uranium [DU] round in the GAU-12 Gatling gun on AV-8 Harriers.
GAU-12 Interceptors have, on rare occasions, been used in a ground attack role against alien forces. Due to this possibility, some UNETCO bases still stock one or two cannons, just in case.