Why have some international armament organizations, despite being designed towards the same goal of producing effective joint armament projects, managed to do so while others have not? This study argues that the institutional design of defence organizations shapes their cooperative armament effectiveness due to the two-level game structure of cooperative arms development and procurement. Institutions can support these structures across national iron triangles of political, military and defence-industrial actors through their degree of centralization and actor-inclusiveness. By constructing a multidimensional, explanatory typology and testing its congruence through the illustrative cases of NORDEFCO and OCCAR, the study highlights the impact of institutional designs on cooperative armament outcomes. It concludes that OCCAR’s centralization of bureaucratic capacities and inclusive design vis-à-vis defence-industrial actors resulted in its highly effective armament outcomes, while the lack of these institutional features can explain NORDEFCO’s dire performance. While further research should focus on expanding the typology and its degree of generalization, this study makes an important contribution to our understanding of the nexus between institutions and defence cooperation. By highlighting that institutions can be precisely structured to increase the effectiveness of joint projects of military force generation, prospective European re-armament efforts could be organized more effectively.