At the time that Dionysus the Elder supposed invented the "crossbow machine gun" (actually called a ballista) which the Mythbusters say could fire 5 bolts per minute, up to 200 yards/600 feet, a trained archer could fire 12 arrows a minute, with an minimum effective range of 400yards/1200 feet.
A soldier armed with a crossbow could also fire an average of 2 bolts a minute and had an average range of 350 yards/1050 feet.
The point of a ballista, a siege engine, was to fire much bigger, heavier bolts, further and faster than a human would be capable of. Ballista bolts were about 3 feet long and weighed approximately 60lbs.