British Forces Hit the Islamic State With ‘the Punisher’ – the Legality of Airburst Munitions

A few days ago, it was reported that British special forces had used a deadly new weapon, nicknamed the Punisher, in a confrontation with ISIL in the Libyan city of Sirte (see here and here (“Who Dares Punishes” in the usual subtle tone of the Sun)). The reports suggest the first operational use by British troops of the XM25, a hand-held weapon firing 25mm anti-personnel rounds that explode in mid-air, directly above enemies hidden behind walls or other obstacles. The use of the weapon brings to mind one of the oldest (and shortest) law of armed conflict instruments, namely the 1868 St. Petersburg Declaration. Adopted at the initiative of the Russian Czar, this instrument specifically condemned the employment of explosive projectiles “of a weight below 400 grams.” Surely then, using the Punisher is diametrically opposed to the prescripts of the St. Petersburg Declaration, an instrument ratified by the United Kingdom and which remains valid today. Or is it?

A game-changer on the battlefield

What sets the XM25 (nicknamed the Punisher) apart from other weapons is its ability to accurately detect and determine range to targets (using a laser rangefinder), and to subsequently program rounds to explode at a pre-determined range. As the first small arms weapon using “smart” technology, the XM25 greatly increases hit probability and lethality when engaging enemies in defilade – whether hiding behind a wall, or in foxholes or ditches (rounds are simply programed to explode above the location where the enemy is hiding). The military advantage is obvious. The weapon also reduces the need for exposure of the soldiers handling the XM25, who may remain behind cover. In addition, the weapon may lead to a reduction in damage, both in terms of human lives and property damage, normally caused by mortars, artillery, and air strikes used to attack targets behind relatively light cover.

The (reported) use of the Punisher by British forces in Libya is not the first occasion the weapon was used on the battlefield – indeed, in the summer of 2010, the US Army began field-testing the XM25 in Afghanistan. It is, however, the first known use of the weapon by a State party to the St. Petersburg Declaration. As a potential game-changer in infantry warfare, the XM25 is likely to increasingly find its way to the battlefield. Various companies and countries are reportedly working on similar weapon systems that fire airburst anti-personnel rounds.

Exploding bullets for anti-personnel use – an absolute prohibition?

At first sight, the use of the XM25 is strikingly at odds with the prohibition against explosive projectiles below a weight of 400 grams laid down in the St. Petersburg Declaration. The easy objection against this reasoning is that the Declaration only prohibits the use of these projectiles “in case of war among” the States Parties to the Declaration, implying that the treaty rule does not apply to the situation in Libya.

Yet, this is only part of the matter. The question arises to what extent the prohibition also forms part of customary international law. It is worth observing in this context that numerous military manuals (including those of Canada and Russia – see here) explicitly copy the prohibition against explosive projectiles below a weight of 400 grams, or contain a general reference to the St. Petersburg Declaration. Yet, before drawing the conclusion that the prohibition is part and parcel of customary international law, it is imperative to look beyond this “verbal” state practice and take into account the “physical” practice of states – in other words, the actual conduct of hostilities.

While a detailed analysis is beyond the scope of this post (a more elaborate analysis is here), it is clear that the original St. Petersburg rule has been significantly eroded as a result of evolutions after 1868, including the use of explosive bullets in aerial warfare, as well as the widespread use of exploding anti-material bullets below 400 grams. Although these evolutions are ignored in the large majority of law of war manuals, the UK Manual, for instance, correctly notes at paragraph 6.12 that the 400 gram limit has become obsolete. Rather than reproduce the St. Petersburg rule, the UK Manual states, more narrowly, that “the use of explosive… bullets designed solely for use against personnel is not permissible under customary law” ( at para. 6.10).

The attentive reader will note that the UK Manual still appears to rule out the use of the XM25, which indeed fires “explosive … bullets designed” for anti-personnel use. Yet, again, the quote does not provide an accurate reflection of the customary residue of the St. Petersburg rule. The better view is indeed that the prohibition only covers the use of impact-triggered single enemy munitions – put differently: bullets that explode on impact with human tissue –, but not airburst munitions, such as those used with the XM25.

In support of this reading, we can also refer to the widespread use of hand grenades and various fragmentation weapons, the permissibility of which is asserted in several military manuals (see, for example, para. 6.11 of the UK Manual). It is noted in this context that the XM25 is commonly referred to as a “grenade launcher”, whereas the US Law of War Manual, for one, refers to it in its section dealing with “Exploding Bullets” (illustrating the difficulty of drawing a clear line between different types of ammunition). In addition, it is worth recalling the raison d’être of the St. Petersburg rule: the aim of that document was to prohibit the use of projectiles causing suffering/mortality to enemy combatants greater than necessary to obtain a legitimate military objective. Put simply: there’s no justification in using bullets that explode within the human body, since a non-exploding bullet would be just as effective and efficient in incapacitating an enemy combatant. That logic does not apply to airburst munitions, whose explosive nature is justified by a clear military advantage, notably the increased probability of hitting enemy combatants in defilade, as well as the possibility of striking multiple combatants simultaneously. These projectiles are indeed not of a nature to cause superfluous injury or unnecessary suffering.

Interestingly, even if the matter is ignored in most military manuals, the United States and the International Committee of the Red Cross (ICRC) appear to be in agreement that the customary prohibition only applies to “impact-triggered” anti-personnel explosive bullets (see also para. 406 of the German manual). Thus, Rule 78 of the ICRC’s customary study on international humanitarian law refers to a prohibition against the anti-personnel use of bullets which explode “within the human body.” The US’s 2015 Law of War Manual acknowledges at paragraph that “bullets that are specifically designed to explode within the human body” contravene the superfluous injury rule. There is of course a major discrepancy between both views (see here for more on this) in that the ICRC, contrary to the US, also discards the anti-personnel use of bullets that have the “effect” of exploding on impact with the human body, even if not “specifically designed” to do so. For present purposes, however, it is sufficient to note the ICRC implicitly acknowledges the legality of airburst munitions for anti-personnel use. The US manual even contains a footnote on page 323 implicitly affirming the XM25’s legality.

In conclusion, in spite of the text of the St. Petersburg Declaration and broadly phrased prohibitions against explosive projectiles below 400 grams or for anti-personnel use in most military manuals, there is no absolute prohibition under customary law against the use of airburst ammunition. Nor is the use of that ammunition likely to contravene the prohibition of unnecessary suffering. Countries, such as the UK, bent on equipping their troops with the XM25 or similar types of weapons, would be well-advised to amend their military manuals to more accurately reflect customary law as it stands.

To a man with a hammer…

The fact that there is no absolute ban of the XM25 under the law of armed conflict is not to say that its use in a particular setting can never be unlawful. Clearly, troops using the weapon must make sure they comply with relevant targeting rules. Thus, if one were to fire XM25 airburst rounds blindly into territory controlled by the enemy, such conduct would arguably contravene the prohibition against indiscriminate attacks. Furthermore, if one were to use XM25 airburst rounds to eliminate an enemy combat hiding behind a wall, in full knowledge that such action might cause harm or even death to multiple innocent bystanders located in the vicinity of the target, this could amount to a violation of the proportionality principle.

While such unlawful use of the XM25 is not a priori excluded, it is recalled that the XM25 may actually be more accurate in a targeting context, and have a lower destructive impact, than some of the alternative weapons that are sometimes used against hostiles in defilade (for example, rifle-mounted “dumb” grenade launchers, mortars or artillery or air strikes). In sum, seen from the perspective of the principle of discrimination and the proportionality principle, the introduction of the Punisher is not necessarily an unwelcome development (even if it could perhaps lead belligerent parties to show less reserve in deciding to engage military objectives with a significant civilian presence).

Still, use of the XM25 may at times be problematic, when less harmful alternatives are available to obtain a certain military objective (e.g., to incapacitate a single and unconcealed human target). The concern in this context is that, as the cliché goes, to man with a hammer, everything looks like a nail. This concern is borne out by the US experience in Afghanistan, where US troops who were asked to try out the XM25 in the field reportedly began using it “as their primary weapon — foregoing additional weapons,” such as the M4 assault rifle. While these instances may not have been problematic, it may be argued that, as a precautionary measure, military units using the XM25 should also be equipped with less harmful alternatives (like the M4) and should be trained to rely on the Punisher only when necessary.

A final issue in this context pertains to the development of non-lethal airburst ammunition – that is, bullets with a smaller explosive charge which will stun opponents rather than kill them. The development of non-lethal or less-than-lethal airburst rounds again brings to the fore the hotly debated issue whether – as the ICRC Guidance on Direct Participation in Hostilities suggests (here, at page 82) – there may be situations where operating forces are obliged to capture rather than kill armed adversaries (see further the excellent analysis by Ryan Goodman). The availability of non-lethal airburst rounds, may all the more have an impact on the assessment of the proportionality and necessity of the recourse to lethal force during counter-terrorist operations beyond zones of active hostilities…

The author wishes to thank Rogier Bartels (ICC) for inspiring the present post. 

About the Author(s)

Tom Ruys

Professor of International Law at Ghent University, Founder of the Ghent Rolin-Jaequemyns International Law Institute (GRILI)