There are MANY things that determine the cyclic rate of fire on a machine gun. So there is no one right answer. Other than they were designed to fire at that rate.
Weight of said parts plays a big factor. Look at an M3 'grease gun'...shoots fast pointed at 90* (3/9 o'clock), faster at 180*(6 o'clock), slower at 0* (12 o'clock). Some weapons have rate reducers to prolong barrel life. heat kills a barrel....design, open bolt fixed firing pin guns shoot faster than hammer fired guns.Springs, speed of gases, length of certain parts, angles of parts that pivot or rotate, etc etc.