The defence industry of the Western Balkans, anchored by Serbia's Yugoslav-era industrial base and by smaller producers in Bosnia and Herzegovina, Croatia, Montenegro, and North Macedonia, has experienced sustained export growth across the 2019–2023 window. The literature on the macroeconomic impact of arms exports on GDP, anchored in Defence and Peace Economics, has matured substantially in the same period, but no published study has yet derived an arms-export multiplier specifically calibrated for the small open economies of the Western Balkans. This article, written with the benefit of the SIPRI Arms Transfers Database update for 2023 and the most recent national accounts releases for the five reference countries, addresses that gap by constructing the Western Balkan Defence-Export Multiplier (WBDEM), a country-specific GDP-elasticity-of-arms-exports metric calibrated to the small-open-economy structure of the region. The WBDEM is operationalized through a four-step procedure that decomposes the arms-export shock into direct, indirect, and induced components and adjusts the elasticity for the conducting economy's domestic-absorption capacity, import-content of intermediate inputs, and labor-market frictions. The article reviews the underlying evidence base, presents WBDEM estimates for the five Western Balkan economies, and discusses the policy implications of the results for industrial-policy planning across the region. Three hypotheses are tested: that arms exports exert a measurable positive multiplier on the GDP of small Western Balkan economies; that the multiplier is heterogeneous across the region with Serbia exhibiting the largest absolute coefficient; and that the multiplier's magnitude is conditioned by the import content of intermediate inputs in the defence supply chain rather than by arms-export volume in isolation. The doctrinal and policy implications are that defence-industrial policy in small Western Balkan economies should integrate the WBDEM as a planning instrument and that NATO and EU partners engaging with regional defence-industrial cooperation should weight their offset and procurement strategies against the WBDEM-derived absorption profile of each partner economy