The Weisenheimer 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.
Despite the low rarity designation of these weapons, their high elemental chance and status effect damage can make them excellent choices for combat.
Other SMGs in this grouping manufactured by Hyperion include the Backburner (incendiary), Storm (shock), and Wellness (slag).
The Weisenheimer is obtained randomly from any suitable loot source.
Because of the fixed parts configuration, SMGs in this group spawn much more rarely than their rarity color would seem to indicate, and may often be overlooked and sold off or ignored by users unfamiliar with their uncommonness. (Maliwan-manufactured SMGs with this barrel are much more common and will spawn at rarities ranging from white to purple.)