The Flurry is built around a hand-held nuclear reactor. When activated, the reactor goes critical, producing enough energy in a short time for practically unlimited plasma shots. Cooling rods snap into place after the burst, preventing a meltdown but rendering the gun inert until it can be serviced outside of the field.
Flurry Guns are lethal ranged weapons which can be used for only one turn per mission. Each attack using a Flurry Gun costs 4 PWR, and deals lethal damage with armor piercing 2. After the turn in which it is used, a Flurry Gun cannot be used again until the next mission. It cannot be recharged using Charge Packs.
A Flurry Gun is a valuable addition to many loadouts, as it can be used to kill armored guards and does not consume expensive Charge Packs. It is the only lethal weapon which does not use ammo. Combined with a Piercing Scanner or Shalem 11's Enhanced Optics, it can pierce armor 3, enough to kill OMNI Soldiers.
The Flurry Gun's major downside is that it can only be used on one turn per mission, and most agents can only make one attack per turn. Killing one guard in an emergency is often enough; however, with forethought and planning (and good other equipment), a Flurry Gun can be used to kill a lot of guards:
In the extreme case, you can kill every guard in the facility, as long as you have enough PWR and you don't mind the high alarm level and exorbitant cleanup cost. You can avoid raising the alarm level by disabling heart monitors with EMP Packs or Dr. Xu's Subdermal Tools in advance. However, even if it takes you to alarm level 6, if you kill all of the other guards, you will be left with just three Enforcers guarding the whole facility, plus any others spawned by Validate or scripted events. It's still best not to kill the Enforcers themselves, because they respawn anyway.