Storm Bolts are a type of ammunition compatible with the StormslingerFW in Horizon Zero Dawn: The Frozen Wilds.
Storm Bolts are electrically-charged projectiles which accumulate higher charge levels with consecutive shots and deal increasing amounts of Explosive damage and Shock severity within a small blast radius. It's current charge level is indicated by five white bars below the aiming reticule which progressively light up as the Bolts are fired uninterruptedly.
The Improved Stormslinger adds three empty bars to the charge meter, enabling Storm Bolts to be overcharged past their maximum safe limit, considerably increasing the damage dealt and size of the blast radius but also backfiring and electrocuting the wielder, with an increasingly bigger loss of health after each overcharged shot. The damage received can be mitigated by wearing Oseram Sparkworker or Shield-Weaver outfits or consuming Resist Shock Potions or Health Boost Potions.