Beags was right:
And the Hunter's 4 x 30mm Aden cannons put more bang into the target than the Phantom's much-vaunted 20mm Vulcan cannon fired for the same length of time!
Just found the AP.
An Aden had a rate of fire of 1200-1600 rpm whereas the SUU was set at 4000 or 6000.
4xAden therefore were delivering between 4800 and 6400 rpm with each shell weighing in at 10 oz.
The weight of the 20 mm is clearly much less.