The rate-of-fire is determined by two factors: one is human capability, the other is the rate that can be taken by the equipment. The rate quoted by PROCON is one that can be sustained for a certain period within the skills/strength/training of the crew. The rate can be exceeded in combat situations but will accelerate tube wear and require more time later for a replacement by ordnance.
That makes sense. I suppose if they had guys lined up on one side holding shells, and guys on the other side holding propellant...they could speed it up.
Looks like a damn tough job to me though. Must make it a little easier to bear when you know people are depending on you with their lives to get those rounds outbound...