Related Rule
Germany
Practice Relating to Rule 70. Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering
Germany’s Soldiers’ Manual (1991) provides: “It is prohibited to use means or methods of warfare which are intended or of a nature to cause superfluous injuries or unnecessary suffering (e.g. dum-dum bullets).” 
Germany, Taschenkarte, Humanitäres Völkerrecht in bewaffneten Konflikten – Grundsätze, Bearbeitet nach ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, Zentrum Innere Führung, June 1991, p. 5.
Germany’s Military Manual (1992) states:
It is particularly prohibited to employ means or methods which are intended or of a nature:
– to cause superfluous injury or unnecessary suffering …
“Superfluous injury” or “unnecessary suffering” is caused by the use of means … of combat whose presumable harm would definitely be excessive in relation to the lawful military advantage intended.
In the 1868 St. Petersburg Declaration the use of explosive and incendiary projectiles under 400 grammes was prohibited, since these projectiles were deemed to cause disproportionately severe injuries to soldiers, which is not necessary for putting them out of action …
It is prohibited to use bullets which expand or flatten easily in the human body (e.g. dum-dum bullets) … This applies also to the use of shotguns, since shot causes similar suffering unjustified from the military point of view. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, §§ 401, 402 and 406-407; see also § 415.
Germany’s IHL Manual (1996) states: “It is prohibited, in particular, to employ means or methods of warfare, which are intended to or of a nature to cause superfluous injury or unnecessary suffering.” 
Germany, ZDv 15/1, Humanitäres Völkerrecht in bewaffneten Konflikten – Grundsätze, DSK VV230120023, Bundesministerium der Verteidigung, June 1996, § 302.
The manual further states:
International humanitarian law prohibits the use of a number of means of warfare which are of a nature to violate the principle of humanity and to cause unnecessary suffering, e.g.
– bullets which easily expand or flatten in the human body, so-called dum-dum bullets,
– weapons whose primary effect is to injure by fragments which in the human body escape detection by X-rays, e.g. plastic or glass ammunition,
– explosive traps, when used in the form of an apparently harmless portable object, e.g. disguised as children’s toys,
– bacteriological means of warfare, e.g. substances which cause disease,
– chemical means of warfare, e.g. poisonous gases. 
Germany, ZDv 15/1, Humanitäres Völkerrecht in bewaffneten Konflikten – Grundsätze, DSK VV230120023, Bundesministerium der Verteidigung, June 1996, § 305.
Germany’s Soldiers’ Manual (2006) states:
It is prohibited to use means or methods which are intended or of a nature,
- to cause superfluous injury or unnecessary suffering (e.g. dum-dum bullets). 
Germany, Druckschrift Einsatz Nr. 03, Humanitäres Völkerrecht in bewaffneten Konflikten – Grundsätze, Erarbeitet nach ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, DSK SF009320187, Bundesministerium der Verteidigung, R II 3, August 2006, p. 5.