A PGU-32/U Semi-Armour Piercing High Explosive Incendiary-Tracer (SAPHEI-T) 25mm round exploded after leaving the fighter's cannon. The accident did not injure the jet's pilot, who was able to bring the aircraft safely back to base.
The mishap was labelled Class A. Class A is the most severe level in the U.S. military's mishap scale, which covers incidents in the air and on the ground. For aviation accidents, Class A mishaps are defined as those resulting in at least $2.5 million in property damage, the total loss of the aircraft, or one or more individuals being killed or permanently disabled.
The F-35 uses a GAU-22 Gatling gun. The GAU-22 is mounted in an external gun pod under the jet's fuselage on the Marines' F-35B and Navy's C-variant.