| This Roman Empire era hand-thrown missile resembles a heavy javelin, and was used primarily as a close-range shock weapon. Pila were usually five to eight feet (one and a half to two and a half meters) long. The wooden shafts were four to five feet (1.2 to 1.5 meters) long, and were fastened to iron heads stretching 14 to 30 inches (36 to 76 centimeters) further along the weapon's length. The heads would end in short (2in/5cm) pyramidal or barbed points with flat or (more rarely) spiked tangs. Some First Century C.E. pila featured a leaden spherical weight behind the point, presumably to improve the weapon's armor-piercing capabilities.
Legionnaires probably threw these weapons at a distance of 20 yards (18 meters), just as they began their charge. The shock of the weapon was likely intended to throw an opposing force off its balance and soften its lines. If a pilum were to only puncture a shield (as opposed to a soldier), it would have still served its function. The shape of the point inhibited quick removal of the weapon, and the iron head prevented its being cut away from the shield; with this heavy javelin embedded in his shield, the enemy soldier would likely find it cumbersome to use. In addition, because the point was tempered and the shaft was not1, the impact would usually bend or break part of the iron shank, making it difficult for the enemy to throw the pilum back. After the battle, legionnaires would be able to retrieve and repair the pila at their leisure.
Evidence also shows that legionnaires carried two pila -- one light and one heavy. The light pilum was suited to throwing as described above, while the heavy pilum featured a thicker shaft and a handguard at the junction of the iron head and wooden shaft. The heavy pilum was more likely used as a standard polearm or lance. These two pila, combined with the pugio, gladius, and scutum, made the legionnaire a deadly weapon for the Roman Empire.
1. With thanks to Ouroboros for reminding me of the pilum's point-tempering. | 
