When I was at the Paul E Garber facility I was able to get right up next to the cockpit glass and I"looked" into the rear cockpit and saw just how the rear guns worked.
Their is a pair of handles "pistol grip style" that are linked a Reflector sight which in turn is mounted to the guns,so when U swing the sight to the right the right gun fires and so on it was a very impressive system(and a religious moment

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Brady