Vegetius, De re militari, 1.17:
"Young
men should also be taught how to use plumbatae, which are also called mattiobarbuli. Two legions in Illyricum, comprising six thousand soldiers each, once wielded these
weapons with such skill and
courage that the
men themselves were called mattiobarbuli. These legions were so successful at winning battle after battle that
Diocletian and
Maximian, upon becoming emperors, ordained that as a reward for their valor these mattiobarbuli should assume the epithets 'Jovian' and 'Herculean' and that they should be considered the
crack troops of the army. Each soldier normally carried five mattiobarbuli attached to
his shield, and, if they threw them effectively, these shield-bearing footsoldiers could virtually fulfill the role of archers. For they could inflict severe damage to enemy soldiers and their horses before it came to hand-to-hand fighting, indeed while the enemy was
still outside the range of javelins." (my translation from the Latin).
See my
thread "S.
Estiot on a
rare bust type of
Probus and the Tetrarchs" at the bottom of p. 1 of Classical
Numismatics