The Storm (SMG) is part of a sub-group of SMGs that only can spawn with a fixed-element Maliwan barrel. They feature the higher-than-usual elemental damage/second and element chance granted by the Maliwan barrel. Because of the fixed parts configuration, SMGs in this group spawn much more rarely than their rarity color would seem to indicate (most often found at green rarity and, even more rarely, blue or purple rarity) and may often be overlooked and sold off or ignored by users unfamiliar with their uncommonness. (Note that Maliwan-manufactured SMGs with this barrel are much more common and will spawn at rarities ranging from white to purple.)
Despite the low rarity designation of these weapons, their high elemental chance and DoT can make them excellent choices for combat until a more suitable or less common (e.g. Legendary or Unique) weapon is obtained.