A potentially useful source of information and insight into USAAF flexible gunnery training:
http://www.ibiblio.org/hyperwar/AAF/...index.html#TOC
ARMY AIR FORCES HISTORICAL STUDIES No. 31
FLEXIBLE GUNNERY TRAINING IN THE AAF
I've read about the first 25 pages or so, and it should give a pretty good account of training problems and methods for the Army Air Forces' efforts during the war.
cheers
horseback