The Rate of fire is the speed at which a specific firearm can operate. It is not technically specific because the same weapon can operate at different speeds due to mechanical reasons and atmosperic conditions. The speed of the fire is measures in rounds per minute. Standard assault rifles have a RPM of 300-400. Machine guns have much higher speeds (1000-1800) and chain guns have even higher rates of fire usually reaching 5 rounds per second.
During history, guns have become speedier. A good example would be the enormious advantage of the Maxim MG , which provided accurate and steady, offensive or supressive fire. This is due to technological advances in fields of material cooling. The construction of the firearm presents a challange because an overheating can occur due to rapid fire.
Famous guns in which the barrell was water cooled (as opposed to air-cooled) were the Browning machine guns.