"The CH-47s are generally armed with a single 7.62-millimeter M60 machine gun on a pintle mount on either side of the machine for self-defense, with stops fitted to keep the gunners from firing into the rotor blades. Dust filters were also added to improve engine reliability. At its peak employment in Vietnam, there were 22 Chinook units in operation. Of the nearly 750 Chinook helicopters in the U.S. and South Vietnam fleets, about 200 were lost in combat or wartime operational accidents." Source: http://en.wikipedia.org/wiki/Boeing_CH-47_Chinook