I do not believe the U.S. had any armed helos in service before or during the Korean War. There are two light attack helicopters in obat12 with availability start dates before the end of the Korean War: the OH-13+ Sioux (Unit 277) and the OH-23+ Raven (Unit 278). I doubt whether either of them were armed until the introduction of the XM-1 system. Moreover, both the OH-13 Sioux and OH-23 Raven were armed with the M2 gun system when they saw combat in Vietnam.
