The pilum (plural pila) was a throwing spear commonly used by the Roman army in ancient times. It was generally about two meters long overall, consisting of an iron shank about 7 mm in diameter and 60 cm long with pyramidal head. The iron shank may be socketed or more usually widens to a flat tang, this was secured to a wooden shaft. A pilum usually weighed between two and four kilograms, with the versions produced during the Empire being a bit lighter. Some versions of the weapon could have been weighted by a lead ball to increase penetrative power, but evidence has not been found yet. Recent experiments have shown pila to have a range of 98 feet (approximately 30 m), although effective range of about half to two thirds.

Legionaries of the Late Republic and Early Empire often carried two pila, with one sometimes being lighter than the other. Standard tactics called for a Roman soldier to throw his pilum (both if there was time) at the enemy just before charging to engage with his gladius. Some pila had small hand-guards, to protect the wielder if he intended to use it as a melee weapon, but it does not appear that this was common.

The late Roman writer Vegetius, in his work De Re Militari, wrote:

As to the missile weapons of the infantry, they were javelins headed with a triangular sharp iron, eleven inches or a foot long, and were called piles. When once fixed in the shield it was impossible to draw them out, and when thrown with force and skill, they penetrated the cuirass without difficulty.^*

And later in the same work:

Their offensive weapons were large swords, called spathae, and smaller ones called semispathae together with five loaded javelins in the concavity of the shield, which they threw at the first charge. They had likewise two other javelins, the largest of which was composed of a staff five feet and a half long and a triangular head of iron nine inches long. This was formerly called the pilum, but now it is known by the name of spiculum. The soldiers were particularly exercised in the use of this weapon, because when thrown with force and skill it often penetrated the shields of the foot and the cuirasses of the horse. The other javelin was of smaller size; its triangular point was only five inches long and the staff three feet and one half. It was anciently called verriculum but now verutum.^*

The small loaded (weighted) javelins to which Vegertius refers are weighted darts called Plumbatae.

Thanks in part to experimental archaeology, it is generally believed that the pilum's design evolved to be armour-piercing: the pyramidal head would punch a small hole through an enemy shield allowing the thin shank to pass through and penetrate a distance sufficient to hit the target. The thick wooden shaft provided the weight behind the punch.

Most later pila were constructed such that the iron shank would bend on impact. A pilum, having penetrated a shield through a small hole and its shank having bent would now be difficult to remove. An enemy, if not killed by the pilum, would have little time before closing with the legionaries and would have to discard his now-unwieldy shield before going into combat. Additionally, bent pila would be less suitable for reuse by a resourceful opponent. Early pila do not seem to have had this characteristic; Gaius Marius is sometimes given credit for this modification. Opinion among archaeologists used to be that the main function of the shank was to disable the pilum by bending, but it is now thought that the pilum was a weapon designed primarily to kill, the 'non-return' aspect being an added bonus.

Ancient Roman legionary equipment | Throwing weapons | Projectiles

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