Si Vis Pacem, 9MM Parabellum. 

Though the bulk of the M3 Grease Guns used in WWII were chambered in .45 ACP, around 1000 M3 submachine guns chambered in 9mm Parabellum were built by Guide Lamp.These original 9mm guns were delivered to the OSS in 1944. The 9mm M3 was also supplied to the French, Belgian, Dutch, Italian and Norwegian resistance groups so that captured German ammo could be used, thus reducing the need for .45 ACP ammo resupply drops from the OSS and the SOE.