Some of the Roman sling bullets found at the Burnswark Hill battle site in Scotland. The two smallest bullets, shown at the bottom of this image, are drilled with a hole that makes them whistle in flight.

Some 1,800 years ago, Roman troops used "whistling" sling bullets as a "terror weapon" against their barbarian foes, according to archaeologists who found the cast lead bullets at a site in Scotland.

Weighing about 1 ounce (30 grams), each of the bullets had been drilled with a 0.2-inch (5 millimeters) hole that the researchers think was designed to give the soaring bullets a sharp buzzing or whistling noise in flight.

The bullets were found recently at Burnswark Hill in southwestern Scotland, where a massive Roman attack against native defenders in a hilltop fort took place in the second century A.D. [See Photos of Roman Battle Site and Sling Bullets]

These holes converted the bullets into a "terror weapon," said archaeologist John Reid of the Trimontium Trust, a Scottish historical society directing the first major archaeological investigation in 50 years of the Burnswark Hill site.

"You don't just have these silent but deadly bullets flying over; you've got a sound effect coming off them that would keep the defenders' heads down," Reid told Live Science. "Every army likes an edge over its opponents, so this was an ingenious edge on the permutation of sling bullets."

Burnswark Hill from the north, with one of the Roman camps visible on the slopes. (Image credit: John Reid/Trimontium Trust)

The whistling bullets were also smaller than typical sling bullets, and the researchers think the soldiers may have used several of them in their slings — made from two long cords held in the throwing hand, attached to a pouch that holds the ammunition — so they could hurl multiple bullets at a target with one throw.

"You can easily shoot them in groups of three of four, so you get a scattergun effect," Reid said. "We think they're for close-quarter skirmishing, for getting quite close to the enemy."

Sling bullets and stones are a common find at Roman army battle sites in Europe. The largest are typically shaped like lemons and weigh up to 2 ounces (60 grams), Reid said.

Smaller bullets shaped like acorns — a symbol the Romans considered lucky — have also been found at Burnswark Hill and other sites in Scotland.

About 20 percent of the lead sling bullets found at Burnswark Hill had been drilled with holes, which represented a significant amount of effort to prepare enough ammunition for an assault, Reid said.

"It's a tremendous amount of work to do, to just chuck them away," he said.

Sling weapon secrets

Whistling sling bullets haven't been found at any other Roman sites, but ceramic sling bullets with holes punched out have been discovered at battle sites in Greece from the second and third centuries B.C, Reid said. [Photos: Ancient Roman Fort Discovered]

Many archaeologists had assumed that the holes in the Greek bullets were reservoirs for poison, he said.

But in slinging experiments using about 100 replicas of the whistling bullets, Reid found that they would have been little use as poisoned weapons.

"The holes are too small, and there's no guarantee that these are going to penetrate skin," Reid said. "And they are ballistically inferior: They don't fly as far, don't fly as fast and don't have the same momentum [as larger sling bullets] — so why put poison holes in only the little ones?"

Reid's brother, a keen fisherman, offered some insight into their possible purpose when he suggested the bullets were designed to make noise in flight.

"I said, 'Don't be stupid; you've no idea what you're talking about. You're not an archaeologist,'" Reid joked. "And he said, 'No, but I'm a fisherman, and when I cast my line with lead weights that have got holes in them like that, they whistle.'"

"Suddenly, a light bulb came on in my head — that's what they're about. They're for making a noise," Reid said.

Deadly in expert hands

At the time of the Roman attack on Burnswark Hill, slings were used mainly by specialized units of auxiliary troops ("auxilia") recruited to fight alongside the Roman legions.

Among the most feared were slingers from the Balearic Islands, an archipelago near Spain in the western Mediterranean, who fought for the Roman general Julius Caesar in his unsuccessful invasions of Britain in 55 B.C. and 54 B.C.

"These guys were expert slingers; they'd been doing this the whole of their lives," Reid said.

In the hands of an expert, a heavy sling bullet or stone could reach speeds of up to 100 mph (160 km/h): "The biggest sling stones are very powerful — they could literally take off the top of your head," Reid said.

Burnswark Hill lies a few miles north of the line of Roman forts and ramparts known as Hadrian's Wall, built during the reign of the emperor Hadrian between A.D. 117 and 138.

Reid said the Roman attack on the Burnswark Hill fort was probably part of the military campaign ordered by Hadrian's successor, the emperor Antoninus Pius, to conquer Scotland north of the wall.

"We think it was an all-out assault on the hilltop, to demonstrate to the natives what would happen to them if they resisted," Reid said.

But the Scottish tribes fought back hard for more than 20 years, and in A.D. 158, the Romans gave up their plans to conquer the north and pulled their legions back to Hadrian's Wall.

"Scotland is rather like Afghanistan in many respects," Reid said. "The terrain is pretty inhospitable, certainly the farther north you go, and the isolation and long supply lines would make it difficult for servicing an army that far north."

Original article on Live Science.