Polybolos

The Polybolos was a Repeating Catapult or Ballista used in Ancient Greece and described by Philon of Byzantium (c.280-220 BCE) as using a chain drive to fire multiple bolts per minute.
It was the Machine Gun of the Ancient World.

Although no Polybolos has ever been discovered, one of the skeletons belonging to a defender excavated at Maiden Castle in southwestern Britain was found to have a bolt in his spine.
It is thought by archeologists to have been fired from a Polybolos during the Roman Conquest of Britannia in 43 CE.

The Polybolos functioned like a modern machine gun.
It used a chain drive to fire multiple bolts per minute.
It used an overhead gravity-fed wooden magazine holding dozens of bolts.
Turning a windlass handle moved a sliding wooden carriage back and forth. This used a flat-link metal chain. the oldest known use of a chain drive in history.
As the operator cranked the winch, the bowstring was pulled back, a new bolt dropped into place from a rotating drum, and the trigger fired the weapon automatically in a continuous cycle.
One bolt or arrow every 5-6 seconds, about 10 per minute, versus 2 per minute by a single shot ballista.
High accuracy. The weapon stayed aimed at the same spot and placed arrow after arrow on exactly the same spot. This was its main drawback, as it didn’t sweep from side to side. Its use was mainly in defending cities against siege towers.

The Chinese invented a Repeating Crossbow during the Warring States Period (475-221 BCE).
This was a hand held crossbow where one hand held a pistol grip and the other drew a lever back and forth which loaded bolts from a magazine and fired them.
It had a short range, little power or accuracy and was ineffective against heavy armour unless the bolt tips were poisoned.