>>71
https://en.wikipedia.org/wiki/M2_Browning#.50_Browning_AN/M2
The M2 machine gun was widely used during World War II, and in later postwar conflicts, as a remote or flexible aircraft gun.
For fixed (offensive) or flexible (defensive) guns used in aircraft, a dedicated M2 version was developed called the ".50 Browning AN/M2" or the "12.7 mm AN/M2".[76]
The "AN" stands for "Army/Navy", since the gun was developed jointly for use by both services.
The AN/M2 designation was also used for other aircraft guns and therefore it is important to write the caliber before the designation.

ブローニングM2を陸海軍共同で航空機に搭載できるように改造したのでAN（Army/Navy）という記号がついた
他にもAN/M2はあるので区別のために ".50 Browning AN/M2" や "12.7 mm AN/M2"というふうに口径をつけいる