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Sweden
Practice Relating to Rule 70. Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering
Sweden’s IHL Manual (1991) considers the “prohibition of methods or means of warfare which cause superfluous injury or unnecessary suffering”, as contained in Article 35(2) of the 1977 Additional Protocol I, as a customary rule of international law. 
Sweden, International Humanitarian Law in Armed Conflict, with reference to the Swedish Total Defence System, Swedish Ministry of Defence, January 1991, Section 2.2.3, p. 18.
The manual further states that, according to the criteria given in the St. Petersburg Declaration and in the 1907 Hague Convention (IV):
Weapons shall be considered particularly inhuman if they:
– cause unnecessary suffering or superfluous damage, or
– have indiscriminate effects, meaning that the weapon effects strike military objectives and civilian persons without any distinction.
These criteria have been used in all arms limitation negotiations in recent years. 
Sweden, International Humanitarian Law in Armed Conflict, with reference to the Swedish Total Defence System, Swedish Ministry of Defence, January 1991, Section 3.3.1, pp. 78–79.
In 1974, during discussions in the Ad Hoc Committee on Conventional Weapons established by the CDDH, Sweden stated:
While it was difficult to discuss the degrees of suffering and injury caused by different weapons, it was not much easier to measure “military advantage” of weapons. Perhaps the gist of the concept was the effectiveness with which a weapon achieved its legitimate task of placing combatants hors de combat. It was not, on the other hand, legitimate military advantage that a weapon caused more or more severe injuries than were needed to disable a combatant …
With regard to weapons which might be deemed to cause unnecessary suffering or superfluous injury, it was hard to see why only civilians should be spared such suffering or injury. The dum-dum bullet had been banned because it caused excessive injury to soldiers. The same ban should apply … to high-velocity small arms projectiles, fléchettes and incendiaries. 
Sweden, Statement at the CDDH, Official Records, Vol. XVI, CDDH/IV/SR.1, 13 March 1974, pp. 11–12, §§ 19 and 23.
In 1977, during a debate in the First Committee of the UN General Assembly, Sweden stated that high-velocity ammunition caused unnecessary suffering and should be banned. 
Sweden, Statement before the First Committee of the UN General Assembly, UN Doc. A/C.1/32/PV.32, 13 October 1977, p. 26.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, Sweden stated:
The use of weapons which cause unnecessary suffering must be considered to be prohibited. The codification of the prohibition of dum-dum bullets was undertaken in accordance with this view, for example. The effect of radioactive radiation as a result of the use of nuclear weapons cause unnecessary suffering, not merely for third parties who are directly affected, but also future generations, for example as a result of genetic damage. 
Sweden, Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 5.
Upon acceptance of the 1995 Protocol IV to the Convention on Certain Conventional Weapons, Sweden declared:
Sweden has since long strived for explicit prohibition of the use of blinding laser which would risk causing permanent blindness to soldiers. Such an effect, in Sweden’s view is contrary to the principle of international law prohibiting means and methods of warfare which cause unnecessary suffering. 
Sweden, Declaration made upon acceptance of Protocol IV to the Convention on Certain Conventional Weapons, 15 January 1997.
In 2007, in an answer to a question in Parliament, Sweden’s Minister of Trade stated: “It is prohibited to use weapons that cause unnecessary injuries and needless suffering to the soldiers that are subjected to them.” 
Sweden, Answer by the Minister of Trade to a Parliamentary interpellation on the sale of cluster munitions to the United States, 13 November 2007, Parliamentary Protocol 2007/08:23, § 18, Anf. 124.
I also share the opinion of the questioner that Sweden should be clear about our view on the use of cluster munitions. When Sweden ratified the Convention on Cluster Munitions earlier this year I stressed that this Convention is important in the efforts to establish a wide ban on cluster munitions and minimize the unacceptable humanitarian suffering that comes from its use. …
There are clear indications that the Syrian regime also uses other weapons that may cause great suffering also among civilians, namely landmines, and the EU have expressed their strong condemnation of the use of these kinds of weapons. 
Sweden, Answer by the Minister for Foreign Affairs to written question 2012/13:149 in Parliament regarding the use of cluster munitions in Syria, 11 December 2012.
Humanitarian law is based on a number of fundamental principles. They are apparent in current treaties and customary law and express the core of humanitarian law. They concern the principles of distinction, proportionality and precaution, the prohibition on causing superfluous damage and unnecessary suffering and the principle of non-discrimination as well as the so called Martens Clause. 
Sweden, Government Bill 2013/14:146 on criminal liability for genocide, crimes against humanity and war crimes, 20 February 2014, p. 33.