The .50 caliber thing is a recurring myth.
From
https://www.tbs.usmc.mil/Pages/Training Corner/sho's/J/BOJ4704 Law of War - Code of Conduct.doc
Small Arms Projectiles. Small arms projectiles, those weighing less than 400 grams (14 ounces) must not be exploding or expanding projectiles. An example of an expanding projectile is hollow point ammunition that is designed to expand dramatically upon impact. Much “mythology” exists about the lawfulness of sniper rifles, .50 caliber machine guns, and shotguns. Bottom line: They are lawful weapons although rules of engagement, commander’s guidance, and tactics may limit their use.
From page 6-79 of TRADOC PAMPHLET 600-4:
CALIBER .50 M2 MACHINE GUN
DESCRIPTION
This weapon provides automatic weapon suppression fire for offensive and defensive purposes. This weapon can be used effectively against personnel, light armored vehicles, and low flying/slow flying aircraft. (emphasis mine)
Sniper rifles, .50 caliber machine guns, and shotguns. Much “mythology” exists about the lawfulness of these weapon systems. Bottom line: they are lawful weapons, although rules of engagement (policy and tactics) may limit their use. (US Army Operational Law Handbook, 1 Jan 2000.)
For example r[sic] many people erroneously believed that the use of a .50 caliber machine gun against individual enemy combatants was a violation of the Law of War. (INTERNATIONAL AND OPERATIONAL LAW DEPARTMENT THE JUDGE ADVOCATE GENERAL'S SCHOOL, U.S. ARMY CHARLOTTESVILLE, VIRGINIA LAW OF WAR WORKSHOP
DESKBOOK)
The use of the .50 caliber machine-gun is perfectly lawful under treaty law and the customary practice of states. (ibid)
Use of .50 caliber weapons against individual enemy combatants does not constitute a violation of this proscription against unnecessary suffering or superfluous injury. (Navy Warfare Publication (NWP) 1-14M, 9.1.1)
50 Caliber legal to use against personnel.(Col. Robert Maguire USMC ,
Laws of War presentation)
For example, many of our soldiers have been taught that the law of war prohibits firing a .50-caliber machine gun at personnel. That is wrong. The law of war does not prohibit the use of .50-caliber or other large caliber weapons against personnel. It is true that many soldiers have been taught otherwise, and because a supposed rule like that does not make sense, it has left these soldiers with a bad feeling about the law of war in general, with a feeling that it ties our hands behind our backs and gets in the way of mission accomplishment. The Office of The Judge Advocate General of the Army recently attempted to discover how this misconception about large caliber weapons made its way into law of war instruction. It appears that the confusion arose from application of the principle of war, economy of force, to the employment of large caliber weapons. Application of the economy of force principle to the use of the .50-caliber machine gun results in the conclusion that it is usually wasteful to employ such weapons against people. This is a weapon intended for bigger targets. Additionally, such wasteful use of the weapon can give away its position in our deployment. Thus, we can see that rules of tactics, not rules of law, dictate the use of the .50-caliber machine gun. (
LAW OF WAR, Subcourse Number MP1023, EDITION B, United States Army Military Police School, Fort McClellan, Alabama, June 1994) (emphasis mine)
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Here is the pièce de résistance (or the coup de grâce, depending on how you look at it). This is from "The Army Laywer", Department of the Army Pamphlet 27-50-172, April 1987, pp. 36-37,
http://www.loc.gov/rr/frd/Military_Law/pdf/04-1987.pdf. It is a scan and as such, doesn't cut and paste properly. I fixed it as best as I could for the time I was willing to invest. However, for the most accurate version, use the preceeding link. All emphasis is mine.
International Law Note
Use of the .50-Caliber Machinegun
One of the recurring myths surrounding the law of war involves a supposed prohibition against the use of the .50 caliber machinegun against enemy personnel. The following opinion, DAJA-IA 1986/8044,21 Nov. 1986, issued by the International Law Division, Office of The Judge Advocate General, dispels this myth, definitively demonstrating that use of the weapon against personnel in the field is consistent with both customary and codified international law:
There is a long history of employment of infantry weapons up to .70 caliber against enemy personnel. The first U.S.musket, made in 1795, was .70 caliber. The first U.S. percussion musket, the Model 1842, was calibber .69, as was an 1847 musketoon developed for use by cavalry, artillery, and sappers. In 1855 the U.S.Army standardized the Caliber 58; the Navy chose to retain the larger caliber .69. Larger wall pieces-up to caliber .75-were manufactured as long range sniper rifles for defense of frontier posts. Muskets and rifles used by other nations during this time also ranged up to .70 caliber.
With the introduction of better grade steel, the breech lock system, rifling, and more powerful propellants, calibers decreased. By 1900, projectiles ranging from calibers .236 to .3 15 had been adopted by the major nations of the world. In contrast with the issue at hand, some argued that this decrease in caliber (and a commensurate increase in muzzle velocity) caused greater suffering than previous larger-caliber weapons, an argument similar to that proffered by Sweden in the 1970s against the 5.56mm (.223 caliber) M-16 rifle. This argument was not supported by medical evidence on either occasion, and was rejected at the Hague Peace Conferences of 1899 and 1907 and the 1978-1980 United Nations Conference on Prohibitions or Restrictions on the Use of Certain Conventional Weapons Which May be Deemed to be Excessively Injurious or to Have Indiscriminate Effects [hereinafter UNCCWJ].
Larger-caliber weapons have remained in the inventories of virtually every nation. For example, the Soviet Union mounts the NSV .50 caliber machinegun on its tanks; it can be removed and employed on a tripod in a ground mode. Nations generally employ 50-caliber machineguns 8s antiaircraft, antimateriel, -and antipersonnel weapons. On occasion, they have been employed specifically as longrange sniper weapons. The Soviet PTRD was a 14.5mm (.58 caliber) bolt-action, single-shot antitank weapon employed during World War II; because of its long-range accuracy, it was frequently employed as a sniper weapon against German troops. Similarly, the Browning Machinegun Caliber .50 HB, M2 currently in use by U.S. forces, was employed as a single-shot sniper rifle during the Vietnam War.
Doctrine for the Browning Machinegun Caliber .50 HB, M2, is contained in U.S.Army Field Manual 23-65 (May 1972). Paragraph 80 provides in part:
Types of targets. Targets presented to the machinegunners during combat will in most cases consist of enemy soldiers in various formations which require distribution and concentration of fire .
a. Point targets are targets which require the use of a single aiming-point. Enemy bunkers, weapons emplacements, vehicles, small groups of soldiers, and aerial targets such as helicopters or descending paratroopers are examples of point targets. . . .
During the 1978 to 1980 UNCCW, as well as at separate conferences of government experts held at Lucerne and Lugano in 1974 and 1976, respectively, discussions of small-caliber weapons included all weapons up to .50 caliber. There were no proposals to restrict the use of the larger small-caliber weapons against personnel. In addition to their universal employment as antipersonnel weapons, there was the practical realization that in firing .50caliber projectiles at other legitimate targets (for example, enemy vehicles), some rounds inevitably would strike exposed enemy personnel. Hence it would have been impossible 'to attempt to limit the intentional attack of enemy personnel with .50 caliber weapons when those personnel could be struck by the same projectiles as the result of the lawful attack of materiel targets.
Employment of the .50 caliber machinegun or other .50 caliberweapons against enemy personnel does not violate the law of war. There remains the question of how the misperception arose as to its purported illegality. There appears one plausible explanation.
During the 1950s, 1960s, and 1970s, the U.S. Army and Marine Corps had in their inventories the M40 106mm recoilless die. Designed primarily for antiarmor use,the M40 was equipped with the M8C .50 caliber spotting gun. The M8C was used to assist the gunner in determining range and leads to the target. It fired a spotter-tracer round containing a tracer element and an incendiary filler. On impact, the incendiary filler produced a puff of white smoke intended to aid in adjusting fire. The spotter-tracer round was designed so that its trajectory matched the trajectory of the 106mm recoilless rifle service ammunition. The spotter-tracer round was designed to be used in the spotting gun only.
Although the M40 could be utilized against enemy personnel (using the flechette-loaded M581 APERS-T round), the M40 essentially was a single shot antitank weapon that relied on concealment and surprise in order to attack enemy armor and survive on the battlefield. Utilization of the M8C .50 caliber spotting gun against an individual soldier would have compromised the position of the M40,making it and its crew vulnerable to attack. Hence tactical, not legal, limitations were placed on the employment of the M8C .50 caliber spotting gun against enemy personnel. It appears that this practical limitation on the use of the M8C somehow was transferred to all 50 caliber weapons, and that in time it was assumed that the restriction was based on some aspect of the law of war. Such transfer of this tactical limitation and the assumption of a law of war basis are incorrect.
Current Army doctrine providing for the use of the .50 caliber machinegun as an antipersonnel weapon is consistent with the law of war obligations of the United States. No treaty language exists (either generally or specifically) to support a limitation on its use against personnel, and its widespread, long-standing use in this role suggests that such antipersonnel employment is the customary practice of nations.
