While it doesn't mention the .50cal this article gives an overview of the problems with free fire zones in Vietnam.
BTW a search of the site doesn't give any hits for .50 caliber. It does mention that there is a difference between a weapon used in aircraft as opposed to by infantry.
It should be noted that a weapon may be unlawful if used for one purpose, and yet lawful if used for another purpose. For example, during World War I, the British armed the machine guns on their warplanes with incendiary bullets. The initial reaction of the German government was to threaten to try captured British airmen as war criminals for violating the St. Petersburg Declaration. Upon reflection, however, the German government backed down. Today, legal experts generally regard the use of incendiary and small caliber explosive bullets as lawful in air warfare, though their use in infantry rifles would still be forbidden. - See more at: http://www.crimesofwar.org/a-z-guide....Kk1cJBQg.dpuf