Modern machine guns are commonly mounted in one of four ways. The first is a bipod – often these are integrated with the weapon. This is common on light machine guns and some medium machine guns. Another is a tripod, where the person holding it does not form a "leg" of support. Medium and heavy machine guns usually use tripods. On ships, vehicles and aircraft machine guns are usually mounted on a pintle mount – basically a steel post that is connected to the frame or body. Tripod and pintle mounts are usually used with spade grips. The last major mounting type is one that is disconnected from humans, as part of an armament system, such as a tank coaxial or part of an aircraft's armament. These are usually electrically fired and have complex sighting systems, for example, the US Helicopter Armament Subsystems.