Related Rule
Practice Relating to Rule 70. Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering
In 1991, in a legal report concerning the withdrawal of its reservation to the 1925 Geneva Gas Protocol, Chile stated:
The prohibition of the use of arms, projectiles or material calculated to cause unnecessary suffering … is considered to be a norm of international customary law and hence to be binding on all States, whether or not they are party to the relevant Convention. 
Chile, Ministry of Foreign Affairs, Memorandum Reservado No. 430/91, Santiago, 17 June 1991, §§ 3 and 6.