Quicksilver

Quicksilver is a firearm. It gives the user an 80% increased chance of breaking a target's appendage(s).

Source
Quicksilver is given to Prompto in Chapter 14. It can later be purchased from the Culless Munitions in Hammerhead for 8,000 gil. It can be sold back to shops for 4,000 gil.

Trivia

 * There are many guns named Quicksilver that appear throughout the Final Fantasy series. Vincent Valentine uses such a gun in FFVII.