This is a primitive firepowder weapon, designed by and popular amongst Beetle artificers. The name comes from its ability to propel its amunition right through a target.
It is a muzzle-loaded design, with multiple barrels firing a projectile about the size of an arrow, but apparently heavier. Presumably the multiple barrels were to allow multiple shots, since reloading is a time-consuming process, but in practice they are often fired in a single, more destructive volley. Stenwold Maker is known to carry and make use of a piercer.