The following firearms are used in the anime FLCL:
Nambu Type 14
Lieutenant Kitsurubami carries two Nambu Type 14 pistols as her sidearm.
In episode 5, a billboard hit by a stay shot during Haruhana's battle with the giant robot depicts a man holding a Beretta 92FS pistol. This is one of several references to John Woo's films throughout the OVA.
Commander Amarao uses a pair of M1911 pistols as his primary weapons.
One of the Interstellar Immigration Bureau agents during episode 5 is armed with a Glock 17 pistol.
During episode 5, anytime Naota gets an idea, it's represented by an x-ray image of a revolver being cocked.
One of Amarao's backup gunmen is armed with a Beretta M12 submachine gun.
Heckler & Koch MP5A3
One of the IIB agents is armed with a Heckler & Koch MP5A3 submachine gun.
Heckler & Koch MP5K
One of the IIB agents is armed with a Heckler & Koch MP5K submachine gun.
One of the IIB agents is seen with an IMI Uzi with the same configuration as Haruhara's airsoft Uzi.
In episode 5, Kitsurubami interrupts the airsoft game by firing at the players with an anti-material rifle of an unidentified make.
Steyr AUG A1
One of the IIB agents is armed with a Steyr AUG A1 assault rifle.
One of the IIB agents is armed with a M16A1 assault rifle.
Haruhara's Rickenbacker 4001 bass guitar is seen being used as something other than a musical instrument on several occasions, including an automatic shotgun (with a rifled barrel) during the gunfight during episode 5.
Several of the agents from the Interstellar Immigration Bureau use weapons that are never seen clearly enough to positively identify or are drawn in a generic style.
A large portion of episode 5, Brittle Bullet, centers around an airsoft game between several of the series protagonists. Given FLCL's tendency towards exaggeration, these replicas are shown ejecting brass and having muzzle flashes with each shot.
Naota Nandaba uses a pair of MAC-11 submachine guns during the airsoft game.
Haruko Haruhara's airsoft gun of choice is an Uzi with a wood stock and handguard.
During the airsoft game, Kanti uses a Franchi SPAS-12 shotgun with the stock removed. In addition to being portrayed as if it were a live-fire weapon, it is seen being fired on full auto (while the real SPAS-12 can only fire either pump-action or semi-auto).
Luger P08 "Artillery" Model
Kamon Nandaba's preferred airsoft weapon is a pair of Luger P08 "Artillery" Model pistols. He also has shoulder stocks attached to them in several scenes.
The giant robot in episode 5 is seen using several gargantuan firearms. As opposed to most mecha in anime that use specialized weaponry, this one uses weapons that are simply regular firearms that are scaled up to match its size.
On a side note, the battle between Haruhara and the robot (especially Harhara's bunny-girl outfit) is a self-referencing parody of the Daicon III and Daicon IV shortfilms that marked the animation debut of Studio Gainax's founders.
Winchester Model 1892
The first weapon that the robot uses is a scaled-up Winchester Model 1892 Saddle Ring Carbine lever-action rifle with a sawed-off stock.
When Haruhara closes the distance between her and the robot, it responds by drawing an over-sized Colt Peacemaker revolver.
When it realizes that it's losing, the robot produces a third weapon, a giant Mauser C96 pistol.
One shot shows Haruhara staring down five different weapons wielded by the giant, the three previously listed weapons as well as two gun barrels that do not appear in any other shot.