ようやく追いついたのだ！引用・参考資料等 https://www.lonesentry.com/blog/grumman-ball-turret.htmlhttps://aviationtrivia.blogspot.com/2010/10/groundbreaking-gun-turret-of-tbf.htmlhttps://www.thoughtco.com/grumman-tbf-avenger-2361509
http://www.nicovideo.jp/watch/sm43685976
