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From Norman Freidman's "U.S. Cruisers", the 6"/47 caliber gun and its rate of fire are discussed both from its designed and actual performance. The gun was new and it fired semifixed ammunition at a designed rate of 10-12 rounds per gun per minute.
When completed and tested during trials:
'it fulfilled expectations: very high volumes of fire (ten rounds or better per gun per minute) were achieved off Guantanamo Bay in March, 1939.
The Savannah's captain reported that the target was "simply smothered".
The Honolulu's wrote that "those who saw this practice said it looked like a stream of bullets playing on the targets, which were almost continuously obscured and drenched with water.'

From James Grace's "The Naval Battle of Guadalcanal", the Helena is described during its initial firing that night.
"Officially the Helena's fifteen six-inch guns fired at a rate of ten rounds per minute at rapid continuous fire, but the ship had reached seventeen. To Lieutenant Luehman, the shooting resembled fifteen fireflies converging on the same spot, or fifteen streams of liquid fire."