Rebellion

Rebellion is a firearm. When the wielder is at 30% HP, the weapon earns a +5 to critical; it earns +1.5 for every percentage under 30% HP.

Source
Rebellion can be purchased from the Verinas Mart in Ravatogh or from the Imperial vendor outside of Steyliff Grove during Chapter 7 for 1,500 gil. It can be sold back to shops for 750 gil.