Long before Pompeii was covered in ash and pumice in the eruption of Mount Vesuvius in 79AD, the people of the doomed city were confronting a threat of a very different kind.

Nearly 170 years earlier, the residents of Pompeii were fighting for their independence when the Roman army, led by General Lucius Cornelius Sulla, attacked the walled city and brought it under its control.

But now Italian researchers said they had discovered that the Romans had a secret weapon - a polybolos - a catapult of Greek design that could launch metal-tipped projectiles at the enemy and knock opposing soldiers off the ramparts in rapid succession.

Researchers Adriana Rossi and Silvia Bertacchi, from the University of Campania Luigi Vanvitelli, and Veronica Casadei, from the University of Bologna, found the unusual weapon was responsible for the large circular craters found in the walls.

The research team recently published their findings in Heritage magazine after five years of engineering research.

The researchers were surprised to find archaeological evidence in the walls on the northern side of the city from 89BC had “miraculously survived” Roman restorations, Second World War bombings and natural disasters.

It left no doubt about the ferocity of the Roman assault on the 40-foot (12-meter) city walls and the critical role that the weapons played in subduing the enemy in the months-long siege.

“The artillery deployed during the assault was primarily intended for anti-personnel purposes, targeting defenders positioned along the ramparts and between the merlons, rather than to demolish the fortifications themselves,” the researchers wrote.

The fan-shaped cavities, well-preserved by the ash released by Vesuvius, had been a source of academic fascination for decades.

The researchers used digital data to create three-dimensional models to mirror the depth, width and shape of the weapon and then compared it with the mechanical motion of a repeating catapult.

The team then reverse-engineered the type of weapon used and found everything pointed to a high-velocity machine – in essence, an ancient machine gun – which pulverised stone on impact.

The researcher said the availability of digital survey techniques had made it possible to record evidence with a level of accuracy and geometric detail that was previously impossible.

Museum collections provided further evidence. Surviving projectiles from other Roman military sites, such as the iron-tipped darts associated with the Scorpion catapult, matched the dimensions of the 3D models created from the damage to the walls.

With a third of Pompeii still buried under volcanic debris from the eruption of Vesuvius, archaeologists are still learning more about the city’s history and how people lived.

Researchers believe more could be discovered about the role of the polybolos as more of the city walls are unearthed.

Try full access to The Telegraph free today. Unlock their award-winning website and essential news app, plus useful tools and expert guides for your money, health and holidays.