In a routine check, German customs officials in Lottstetten seized the weapon, citing its owner’s lack of a gun licence, newspaper Südkurier reports.
The main customs office in Singen noted that such rudimentary weapons, usually homemade, are often fashioned from metal or plastic tubes, with gas used to propel the missiles.
Despite the name, spud guns can also be used to fire apples, golf balls, or even tennis balls at speeds of up to 250 kilometres per hour.
Aimed at people or animals, potato guns can cause severe injuries.
The owner of the "spudzooka", who lives in Switzerland, was only permitted to drive on once he had paid the €300 fine.