June 2015 U.S. Law of War Manual
The US Law of War manual is a compilation of all of the treaty requirements pertaining to war that the U.S. has agreed to abide by. I do have the latest version in searchable format and beginning on Page 317 is the section relevant to weapons and ammunition. I’m going to abstract portions relevant to this thread. Especially significant to this thread is footnote (79) below
==
6.5.2 Other Examples of Lawful Weapons. In particular, aside from the rules prohibiting weapons calculated to cause superfluous injury and inherently indiscriminate weapons,(46) there are no law of war rules specifically prohibiting or restricting the following types of weapons by the U.S. armed forces:
…
• small arms, cannons, and other guns, including shotguns, exploding bullets, expanding bullets, suppressors, or large-caliber guns;(48)
…
6.5.4 arms, cannons, and other guns are not specifically restricted or prohibited by the law of war. In particular, there are no specific rules against shotguns, exploding bullets, expanding bullets, suppressors, or anti-personnel use of larger caliber guns or cannons.
…
6.5.4.4 Expanding Bullets. The law of war does not prohibit the use of bullets that expand or flatten easily in the human body. Like other weapons, such bullets are only prohibited if they are calculated to cause superfluous injury.(74) The U.S. armed forces have used expanding bullets in various counter terrorism and hostage rescue operations, some of which have been conducted in the context of armed conflict.
The 1899 Declaration on Expanding Bullets prohibits the use of expanding bullets in armed conflicts in which all States that are parties to the conflict are also Party to the 1899 Declaration on Expanding Bullets.(75) The United States is not a Party to the 1899 Declaration on Expanding Bullets, in part because evidence was not presented at the diplomatic conference that
expanding bullets produced unnecessarily severe or cruel wounds.(76)
In 2013, a review conducted by DoD in coordination with the Department of State reconfirmed that the prohibition in the 1899 Declaration on Expanding Bullets did not reflect customary international law.(77) The findings of this review were consistent with the longstanding position of the United States not to become a Party to the 1899 Declaration and not to apply a distinct prohibition against expanding bullets, but instead to regard expanding bullets as prohibited only to the extent that such bullets are calculated to cause unnecessary suffering.(78)
In the past, expanding bullets have been viewed as ineffective for military and technical reasons.(79) Non-expanding bullets that were widely used in armed conflict produced wounds comparable to, or more severe than, wounds produced by expanding bullets.(80) In addition, expanding bullets are widely used by law enforcement agencies today, which also supports the conclusion that States do not regard such bullets are inherently inhumane or needlessly cruel.(81)
6.5.4.5 Rome Statute of the International Criminal Court War Crime Regarding the Use of Expanding Bullets. The war crime of using expanding bullets that is reflected in the Rome Statute of the International Criminal Court has been interpreted by States only to criminalize the use of expanding bullets that are also calculated to cause superfluous injury and not to create or reflect a prohibition against expanding bullets as such.
The Rome Statute of the International Criminal Court lists as a war crime “[e]mploying bullets which expand or flatten easily in the human body, such as bullets with a hard envelope which does not entirely cover the core or is pierced with incisions.”(82)
Elements of Crimes have been adopted by the Assembly of States Parties to the Rome Statute to assist the ICC in its interpretation and application of the Article 8 of the Rome Statute, which includes this war crime.(83) The Elements of Crimes explain that this rule is not violated unless, inter alia, “[t]he perpetrator was aware that the nature of the bullets was such that their employment would uselessly aggravate suffering or the wounding effect.”(84) When adopting an amendment at the Review Conference in 2010 that would apply this crime to non-international armed conflict, Parties to the Rome Statute reiterated this understanding and explained that “the crime is committed only if the perpetrator employs the bullets to uselessly aggravate suffering or the wounding effect upon the target of such bullets, as reflected in customary international law.”(85) Thus, this crime only applies to expanding bullets that are also calculated to cause superfluous injury and does not create or reflect a prohibition against expanding bullets as such.(86)
Relevant footnotes
(79): 79 See, e.g., Alfons Vanheusden, W. Hays Parks, and William H. Boothby, The Use of Expanding Bullets in Military Operations: Examining the Kampala Consensus, 50 MILITARY LAW AND LAW OF WAR REVIEW 535, 537 (2011) (“[L]ack of use of expanding ammunition by armed forces could be attributed equally to the increased risk of weapon malfunction (specifically, failure to feed) through its use, particularly in machineguns”); P. R. COURTNEYGREEN, AMMUNITION FOR THE LAND BATTLE 33 (1991) (noting that expanding bullets “may not cause a sufficiently incapacitating wound, particularly in a military context,”); WILLY LEY, SHELLS AND SHOOTING 39-41 (1942) (“[D]umdum bullets find too much air resistance. Roughly speaking, their power of penetration is only one-quarter that of other bullets, and the ‘dumdum effect’ does not take place if the velocity is low. Ranges of more than 600 yards ruin the ‘dumdum effect,’ and over 1200 yards the bullet may be so slow that it cannot penetrate a heavy uniform with overcoat.”); Charles Frederick Carter, “Atrocities” in War, Nov. 1914, 29 THE WORLD’S WORK 65, 66 (1915) (“But since the first Hague Conference no nation has used dum-dum bullets for a compelling reason that has nothing to do with the Hague nor with considerations of humanity. Experience has taught that when a modern high powered rifle, such as is used in all armies to-day, is hot and dirty, conditions common to battle, the dum-dum bullet is liable to ‘strip’; that is, the leaden core is apt to squirt out, leaving the jacket in the barrel, so that when the next shot is fired the gun blows back, or bursts.”).
===
and there you have the official position of the US military as of June 2015. No prohibition - just military effectiveness.