Rate of fire is the frequency at which a specific weapon can fire or launch its projectiles. This can be influenced by several factors, including operator training level, mechanical limitations, ammunition availability, and weapon condition. In modern weaponry, it is usually measured in rounds per minute (RPM or round/min) or rounds per second (RPS or round/s). There are three different measurements for the rate of fire: cyclic, sustained, and rapid. Cyclic is the maximum rate of fire given only mechanical function, not taking into account degradation of function due to heat, wear, or ammunition constraints. Sustained is the maximum efficient rate of fire given the time taken to load the weapon and keep it cool enough to operate. Finally, rapid is the maximum reasonable rate of fire in an emergency when the rate of fire need not be upheld for long periods. Overview For manually operated weapons such as bolt-action rifles or artillery pieces, the rate of fire is governed primarily by the training of the operator or crew, within some mechanical limitations. Rate of fire may also be affected by ergonomic factors. For rifles, ease-of-use features such as the design of the bolt or magazine release can affect the rate of fire. For artillery pieces, a gun on a towed mount can usually achieve a higher rate of fire than the same weapon mounted within the cramped confines of a tank or self-propelled gun. This is because the crew operating in the open can move more freely and can stack ammunition where it is most convenient. Inside a vehicle, ammunition storage may not be optimized for fast handling due to other design constraints, and crew movement may be constricted. Artillery rates of fire were increased in the late 19th century by innovations including breech-loading and quick-firing guns. For automatic weapons such as machine guns, the rate of fire is primarily a mechanical property. A high cyclic firing rate is advantageous for use against targets that are exposed to a machine gun for a limited time span, like aircraft or targets that minimize their exposure time by quickly moving from cover to cover. For targets that can be fired on by a machine gun for longer periods than just a few seconds the cyclic firing rate becomes less important. For a third hybrid class of weapons, common in handguns and rifles, known as a semi-automatic firearm, the rate of fire