The Mk IV was having trouble with it's S-guns, there's a bug in the system regarding cannons and gun pods; they only seem to shoot rifle calibre rounds for some reason (unless someone has discovered a work around for this). The MAW IID doesn't use the S-guns as a seperate loadout, hence no problems...
Winjeel
Hiya Winjeel,
the fix is to build the weapons into the aircraft model.
Do not attach them as a selectable payload.