I think it's 20mm Vulcan in the F-16s.
At some point, RSAF did have both 30mm Adens and the 20mm M20(?) on
the A4S, Hawker Hunters and F5s.
Later they consolidated and removed the 30mm Adens on the A4s to
the 20mm calibre.
I think the army might want to relook at this:-
too many ammo types......