The FBI-9 is a fictional gun designed by Krinkels seen only on two occasions in Madness Combat 5: Depredation - once in the cabinet where Hank obtained two Micro Uzis and again on the posters advertising the weapon. According to the poster, the FBI-9 is an assault rifle that fires an 11x72mm short cartridge and has a 40mm grenade launcher equipped as a standard attachment.
In order to enforce the comedic nature of the gun, the poster also stated that there is an "optional water balloon launcher attachment." The poster also notes that the FBI-9 is "legal in 23 states, mostly the south". This is a reference to the southern states of the U.S.A. having more lenient firearm laws.