Old-style Hyperion SMGs have higher damage output at the cost of significantly lower accuracy and fire rate than their modern analogs. They also have a limited version of Hyperion's "reverse recoil" when aimed down the sights only.
The Brainstorm 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 what can make them excellent choices for combat until a more suitable or more accurate weapon is obtained. These parameters achieve even higher values, if the SMG is equipped with a Maliwan grip.
Other SMGs in this grouping manufactured by the "old" Hyperion include the Ascerbic (corrosive), Boiler (incendiary), and Fridge (cryo).
The Brainstorm only comes in green or higher rarity.
In addition to the Brainstorm, a variant of this weapon exists as the Storm, made by the "new" Hyperion.
The Brainstorm is obtained randomly from any suitable loot source.