The Shockstorm is a unique variant of the GP-102 rifle. It was manufactured by the Midnight, and thus has the advantages of their manufactory skills.
The Shockstorm retains the sleek black look of the GP-102 rifle, but has a collection of small tesla coils haphazardly forged onto the right side of the receiver. The weapon is also made out of Lunarium, allowing the weapon to heal itself but weigh considerably more than it's common counterparts (the Shockstorm weighs 35lbs while the GP-102 weighs 25lbs).
While the GP-102 fires a high-power shot fast enough to create a Sonic Rainboom (hence it's nickname, the "Rainboom Rifle"), the Shockstorm instead creates a unique electrical storm along the bullet's flightpath that lingers and causes an EMP effect to targets that get caught in the storm. The bullet itself does tremendous damage (able to destroy a fully-armoured Legion in one shot through the Gun Shield) and also causes an EMP blast around the impact zone, as well as creating a lingering electrical storm around the point of impact. Before any of this can be done, the gun must first "charge" an electrical current which causes an electrical disturbance around the user and around the weapon itself, causing an EMP effect on the wielder. The weapon has exceedingly high recoil, even more so than the standard GP-102, forcing the user to take a prone and stable position before firing.
The weapon's exact origins are unknown, though Calamity is in possession of it when it is first shown. Upon Calamity's execution at the hands of Scootaloo, the weapon was taken by Union Strike and eventually used during the Everfree Skirmish to disable the Enclave's three Thunderhead ships so they could be shot down.