The javelin, or pilum, consists of a long iron head with a small point, and a wood shaft. On the most common type, the bottom of the head widens into a flat tang, which is riveted into the widened top of the wood shaft. The second type has a socketed head, and a third type, less well-known, has a spike tang. In the first Century AD, some tanged pila are shown with a spherical weight, presumed to be lead, behind the joint block. Apparently the weapon had become lighter over the centuries, and the weight was added to increase its "punch".
Pilum heads are 14" to 30" long, with pyramidal or barbed points approximately 2" long. The iron shanks are about 1/4" thick near the point, swelling to approximately 1/2" at the base. The wood shaft is 7/8" to 1 1/8" in diameter and 4 to 5 feet long, making the complete weapon 5 1/2' to 7 1/2' in length. The butt is capped with a simple iron cone.
The pilum was probably thrown at a range of 20 yards, just as the Roman line charged. The small point could penetrate a shield and wound the man behind it, or even pierce armor. Any man with a pilum stuck in his shield would find the javelin's weight so cumbersome that he would probably discard the shield; the pilum's head shape prevented its easy removal, and the iron shank prevented its being cut off. (This shield-removing capability has always been over-emphasized; the pilum was designed to kill.)
Finally, no matter what the javelin hit, its unhardened iron shank was supposed to bend, if only a little, so that an enemy could not throw it back. When the Romans were finished winning the battle they could gather their pila and straighten them.
During the Republic, each legionary carried two pila, one light and one heavy. Most illustrations of Imperial legionaries show only one pilum, but at least one shows two, both tanged and apparently identical. It would appear that two pila were still carried, but there was no longer any differentiation between "heavy" and "light".