Penetrator is a Legendary weapon obtainable from any mob (except Statues) with a 1:25000 drop chance.
Shoots fast-moving bullets.
Critical hits damage varies from 3x to 5x.
Uses your health instead of ammo.
It is a Legendary ranged weapon, which means that its stats will grow with every boss killed, including some of the modded ones, for example, bosses from Calamity mod.
Please note, that killing same bosses over and over again will not yield you the upgraded stats.