Did some Chinooks become attack helicopters in Vietnam?

The ACH-47A was first known as the Armed/Armored CH-47A before receiving the official designation of US Army attack and cargo helicopter. Boeing Vertol converted four CH-47A helicopters in late 1965, adding armor and upgraded engines. The armament was formidable for a helicopter of its time: two fixed forward-firing M24A1 20 mm cannons, a nose turret with an automatic 40 mm grenade launcher, five .50 cal (12.7 mm) machine guns, and two side pods for additional weapons.

Three of the four aircraft were assigned to the 53rd Aviation Detachment in South Vietnam, while the fourth stayed in the United States for weapons testing. They arrived in 1966 and spent six months of operational evaluation at An Khe Army Airfield. In combat they performed well, but their maintenance was expensive, and the Army needed Chinooks more urgently as troop and cargo transports.

Three ACH-47s were lost. One collided with another CH-47 while taxiing on the ground. Another crashed when a retaining pin on one of its 20 mm cannons came loose and the weapon fired into the forward rotor blades. The third was disabled by enemy fire and destroyed. Unofficially, soldiers called these gunships "Guns A Go-Go." The experiment never went into full production, but it demonstrated the versatility of an airframe designed to move troops, artillery, and supplies.