The insurgents, often concealed in palm groves, lie in wait for the helicopters and then attack the helicopter, usually from the rear. The weapons used include rocket-propelled grenades and heat-seeking shoulder fired missiles such as the SA-7, SA-14, and in one case the SA-16. Countermeasures taken by helicopter pilots, such as flying very low at a high speed, have considerably reduced the number of helicopters shot down, by reducing the time of target acquisition. Recently, the tactic of flying low has increased the vulnerability of these vehicles to .50 caliber machine gun fire. Helicopters, including Apache gunships, have been severely damaged or destroyed when multiple machine gunners have engaged helicopters at close ranges of 50–400 meters. At this range, the kinetic energy of these bullets is sufficient to penetrate the helicopter's armor.