is primarily governed by the ability of the operator to actively pull the trigger and for aimed fire the operator's shot-to-shot recovery time. 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. Over time, weapons have attained higher rates of fire. A small infantry unit armed with modern rifles and machine guns can generate more firepower than much larger units equipped with older weapons. Over the 20th century, this increased firepower was due almost entirely to the higher rate of fire of modern weapons. A good example of growth in rate of fire is the Maxim machine gun that was developed in 1884 and used until World War I ended in 1918. Its performance was improved during that time mainly by advances in the field of cooling. Measurement There are diverse measurements of rate of fire. The speed of the fire will vary depending on the type of automatic weapon. Cyclic rate This measures how quickly an automatic or semi-automatic firearm can fire a single cartridge. At the end of a cycle, the weapon should be ready to fire or begin firing another round. In an open bolt simple blowback weapon, this starts with pulling the trigger to release the bolt. The bolt pushes a cartridge into the barrel from a magazine and fires it. The energy propelling the bullet also pushes the bolt rearward against the recoil spring. After the bolt is stopped by either the spring or the rear of the receiver, it is pushed forward to either fire again or catch on the sear. Typical cyclic rates of fire are 600–900 rpm for assault rifles, 400–900 rpm for submachine guns and machine pistols, and 600–1,500 rpm for machine guns. M134 Miniguns mounted on attack helicopters and other combat vehicles can achieve rates of fire of over 100 rounds per second (6,000 rpm). Effective rate This is the duration of firing that a weapon could be expected to realistically withstand or output in a realistic environment. On paper, the