The Zwei Fasser is a double-barreled shotgun available to the player about mid-way through the game. It is described in Sheriff Bartlett's Journal as being a piece of "old world" workmanship, and despite being sketched with a ramrod, appears to be a breech loader, as evidenced when Red is inserting shells into it.
The Zwei Fasser is one of the more powerful shotguns in-game, capable of taking out a small bunch of enemies at a fairly decent range.
Zwei Fasser is German for "Two Barrels". While "Fass" is the translation of "barrel", the German word is really only used for the liquid container. The correct translation would be "Zwei Läufe" or "Zwei Rohre".
Despite being described as an older make of shotgun, the Zwei Fasser is hammerless, something that was not prevalent among older model weapons.