Several different measurements are used. The fastest and most commonly quoted rate is the cyclical rate of fire. However, heat (possibly leading to weapon failure) and exhaustion of ammunition mean most automatic weapons are unlikely to sustain their cyclic rate of fire for a full minute. That is why lower rates apply in practice and it is incorrect to describe the cyclic rate of fire as "the number of rounds a weapon can fire in one minute."
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. 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 is primarily governed by the ability of the operator to actively pull the trigger. No other factors significantly contribute to the rate of fire. Generally, a semi-automatic firearm automatically chambers a round using blowback energy, but does not fire the new round until the trigger is released to a reset point and actively pulled again. Semi-automatics' rate of fire is significantly different from and should not be confused with full-automatics' rate of fire. Many full-automatic small arms have a selective fire feature that 'downgrades' them to semi-automatic mode by changing a switch.