United States of America
Practice Relating to Rule 77. Expanding Bullets
The US Field Manual (1956) states: “Usage, has … established the illegality of … the scoring of the surface or the filing off of the ends of the hard cases of bullets.” 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 34(b).

The US Air Force Pamphlet (1976) states:
International law has condemned dum dum … bullets because of types of injuries and inevitability of death. Usage and practice has also determined that it is per se illegal … to use irregularly shaped bullets or to score the surface or to file off the end of the hard cases of the bullets which cause them to expand upon contact and thus aggravate the wound they cause. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-3(b)(2).

The US Instructor’s Guide (1985) stresses the prohibition of “irregular-shaped bullets such as dum-dum bullets”. 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 7.

The Guide also provides: “In addition to the grave breaches of the Geneva Conventions, the following acts are further examples of war crimes: using … forbidden arms or ammunition such as dum-dum bullets.” 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 13.

In 1974, in reply to a letter from a member of the US House of Representatives, the Acting General Counsel of the US Department of Defense stated:
The United States is not a party to the agreement prohibiting the use of expanding bullets or “dum-dums”, signed at The Hague, July 29 1899. In that Agreement, the parties agreed “to abstain from the use of 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”. The United States has, however, acknowledged that it will abide by the terms of the agreement prohibiting expanding bullets. 
United States, Department of Defense, Acting General Counsel, Reply of 18 January 1974 to a letter of 14 November 1973 of a member of the House of Representatives, Arthur W. Rovine, Digest of United States Practice in International Law, 1974, Department of State Publication 8809, Washington, D.C., 1975, pp. 705–706.

At the CDDH, the United States voted against the Philippine amendment (see supra) because:
Grave breaches were meant to be the most serious type of crime; Parties had an obligation to punish or extradite those guilty of them. Such crimes should therefore be clearly specified, so that a soldier would know if he was about to commit an illegal act for which he could be punished. The amendment, however, was vague and imprecise. What standard would be applied, for example, in deciding whether a bullet expanded or flattened “easily” in the human body? … It would also punish those who used the weapons, namely, the soldiers, rather than those who made the decision as to their use, namely, Governments. 
United States, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.44, 30 May 1977, pp. 280–281, § 7.

In 1983, in a memorandum on the use of small-caliber armor-piercing incendiary (API) ammunition against enemy personnel, the US Department of the Army emphasized that no US ammunition violated, inter alia, the 1899 Hague Declaration concerning Expanding Bullets. 
United States, Department of the Army, Office of the Judge Advocate General, Memorandum on the use of small-caliber armor-piercing incendiary (API) ammunition against enemy personnel, 15 March 1983, § 2.

In 1990, in a memorandum of law on sniper use of open-tip ammunition, the US Department of the Army stated:
The United States is not a party to [the 1899 Hague Declaration concerning Expanding Bullets], but U.S. officials over the years have taken the position that the armed forces of the United States will adhere to its terms to the extent that its application is consistent with the object and purpose of article 23e of [the 1907 Hague Regulations]. 
United States, Department of the Army, Office of the Judge Advocate General, Sniper Use of Open-Tip Ammunition – Memorandum of Law, 12 October 1990, p. 4, § 3.

He added, however:
Wound ballistic research over the past fifteen years has determined that the prohibition contained in the 1899 Hague Declaration [concerning Expanding Bullets] is of minimal to no value, inasmuch as virtually all jacketed military bullets employed since 1899 with pointed ogival spitzer tip shape have a tendency to fragment on impact with soft tissue, harder organs, bone or the clothing and/or equipment worn by the individual soldier.

Weighing the increased performance of the pointed ogival spitzer tip bullet against the increased injury its break-up may bring, the nations of the world – through almost a century of practice – have concluded that the need for the former outweighs concern for the latter and does not result in unnecessary suffering as prohibited by the 1899 Hague Declaration Concerning Expanding Bullets and the 1907 Hague Convention IV. The 1899 Hague Declaration Concerning Expanding Bullets remains valid for expression of the principle that a nation may not employ a bullet that expands easily on impact for the purpose of unnecessarily aggravating the wound inflicted upon an enemy soldier. 
United States, Department of the Army, Office of the Judge Advocate General, Sniper Use of Open-Tip Ammunition – Memorandum of Law, 12 October 1990, pp. 6–7, § 6.

In 1990, in a memorandum of law on sniper use of open-tip ammunition, the US Department of the Army stated that the use of the 7.62 Norma Match ammunition with open-tip bullet is not contrary to the Hague or Geneva rules, since the
purpose of the 7.62mm “open-tip” MatchKing bullet is to provide maximum accuracy at very long range … It may fragment upon striking its target, although the probability of its fragmentation is not as great as some military ball bullets currently in use by some nations. Bullet fragmentation is not a design characteristic, however, nor a purpose for use of the MatchKing by U.S. Army snipers. Wounds caused by MatchKing ammunition are similar to those caused by a fully jacketed military ball bullet, which is legal under the law of war … The military necessity for its use … is complemented by the high degree of discriminate fire it offers. 
United States, Department of the Army, Office of the Judge Advocate General, Sniper use of open-tip ammunition – Memorandum of law, 12 October 1990, p. 7.

In 1993, in a legal review of the USSOCOM Special Operations Offensive Handgun, the US Department of the Army stated:
The Hague Declaration Concerning Expanding Bullets of 29 July 1899 prohibits the use in international armed conflict … of 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.
The United States is not a party to this declaration, but has taken the position that it will adhere to the terms of this convention and its conventional military operations to the extent that its application is consistent with the object and purpose of article 23e of the [1907 Hague Regulations].

The conflict spectrum clearly has changed from 1899, and the immediate incapacitation essential to the prevention of the release of dangerous materials or the murder of hostages or prisoners of war necessitates reconsideration of the 1899 prohibition in light of these changed circumstances. The Hague Declaration retains its general validity in limiting use of expanding ammunition by conventional military forces in conventional armed conflict when such use may result in superfluous injury, absent a clear showing of military necessity for its use. 
United States, Department of the Army, Office of the Judge Advocate General, Legal Review of USSOCOM Special Operations Offensive Handgun, 16 February 1993, pp. 12 and 17.

In 1996, in a legal review of the Fabrique Nationale 5.7x28mm Weapon System, the US Department of the Army stated:
The United States is not a party to [the 1899 Hague Declaration concerning Expanding Bullets], but has taken the position that it will adhere to the terms of the convention in armed conflict to the extent that its application is consistent with the object and purpose of article 23e of the [1907 Hague Regulations]. 
United States, Department of the Army, Office of the Judge Advocate General, Fabrique Nationale 5.7x28mm Weapon System: Legal Review, 13 May 1996, p. 3.