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.
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".
The Zwei Fasser is the only shotgun in the game to be hammerless, a design which first appeared around the 1880s in Europe.