In the anime Neon Genesis Evangelion, the pallet gun (also called pallet rifle) is the large rifle-like weapon used by the Evangelions against the angels. The name of this weapon is almost certainly either a typo or Engrish for pellet gun, but the large, high-explosive rounds it fires can hardly be termed 'pellets'. It is used by EVA-01 against the fourth, seventh and ninth angels, and by EVA-02 against the 14th angel, Zeruel. EVA-00 wields, but never fires, this weapon when fighting Bardiel, the 13th angel.
The weapon itself is a large shoulder-fired device with a shrouded barrel (contained entirely within the housing of the weapon, save for a small flash suppressor at the muzzle). It is a fully automatic firearm, discharging roughly 3-7 rounds per second. As the weapon does not seem to eject spent shells, it would appear to use caseless ammunition. Judging from the width of the barrel relative to the size of an Eva, the caliber of the weapon can be estimated at approximately 155mm. Delivering this kind of cyclic rate from a 155mm cannon is probably outside our current technology. That said, this is science-fiction, or arguably fantasy we're talking about here, so practical feasibility is probably irrelevant.