In the performance of law enforcement tasks, military forces frequently use expanding bullets. Such bullets are prohibited in international armed conflicts (IAC:s) by treaty, but in non-international armed conflicts (NIAC:s), the matter is regulated by the principle prohibiting means and methods of a nature to cause superfluous injury or unnecessary suffering (SIrUS), and possibly by an independent rule of customary international humanitarian law. This essay looks first to a proposed solution in which the law enforcement legal paradigm takes precedence, and finds such a solution incomplete and its application limited. It proceeds to look at the prohibition of expanding bullets in NIAC:s as an independent rule,exploring the formation of customary law, the evidence value of military manuals, expanding bullets as a war crime, and the ICTY Tadíc ruling. It finds the evidence of the existence of such a rule unconvincing. It then looks to principle of SIrUS and explores the best approaches for its interpretation and application. Ultimately, it argues that the military utility, in the form of stopping power and decreased risk of collateral injury, provided by expanding bullets is of such a scale and nature that the use of such bullets could reasonably be argued to pass the assessment as required by the principle. In light of this, the essay emphasises the need for treaty-based rules in order create effective weapons prohibitions, and that armed forces are still obliged to properly assess which set of rules govern the use of force.