The AV-8B uses a 25mm gun in a pod installed on the left hand side of the fuselage.  It shakes the living crap out of everything mounted adjacent to and forward of the muzzle.  I don't recall the number now, by we sized everything in the forward fuselage on that side with a giant  static inertia load and a severe fatigue spectrum in hopes of preventing structural or equipment failure.

The UK uses a GR designation for their AV-8B's.  GR-MK VII is the highest effectivity I remember working on.

The airplane has no room for an internal gun.  The pods are installed in place of the lift fences.