The Panzerschrek is a rocket launcher available only from the mystery box. It has a one rocket capacity, with four rockets in reserve. The Pack-a-Punched version is the Longinus, which features a three rocket capacity and 30 rockets reserve. It also has increased damage.
The Longinus is not refilled by max ammo's.
The weight of the launcher(s) makes the player so slow, they can hardly run frum the zombies, even when sprinting.
The Panzershrek is not in the 1.2 Alpha, though the sound files are present.