The pilum - the Roman spear
The pilum was thrown just as the legionaries charged. The small bullet shaped metal point could penetrate shields and armour and cause serious injury to the enemy.
In the event of it striking a shield but not causing injury, the shape of the tip made it very difficult to remove. The shield now became impossible to manoeuvre, due to the weight of the wooden shaft, and would very likely be discarded - leaving its owner vulnerable at a critical moment.
Finally, because of the narrowness of the soft iron shank and the weight of the wooden shaft, the shank bent on impact. This meant that the enemy was not able to throw it back at its owner. However, after the battle, the Romans could collect the 'used' pila for their blacksmiths to straighten.