M.C. Bishop 
Pilum [PDF ebook] 
The Roman Heavy Javelin

A heavy javelin, normally used as a shock weapon immediately before contact, the pilum was designed with a particular speciality: it could penetrate a shield and carry on into the individual behind it. Relying on mass rather than velocity, at short range a volley of pila had much the same effect on a charging enemy as musketry would in later periods. The design was not uniform, with a wide diversity of types throughout the developmental history of the weapon, but for more than four centuries it remained a vital part of the arsenal of weapons at the disposal of the Roman legionary. Drawing upon recent major finds in the Iberian Peninsula and the Balkans, as well as written records and rigorous scientific analysis, this enthralling study lifts the veil on the evolving nature of the pilum, the Roman heavy javelin that helped to conquer the known world.