After War Gundam X (also known as "Mobile New Century Gundam X") is the seventh television series in the Gundam franchise (dating back to 1979) and the third not set in the franchise's usual U.C. timeline. Set fifteen years after a war between the Earth Government and the rebelling space colonies results in 99% of humanity being wiped out, the series follows a group of salvage dealers (called "Vultures") who find themselves entangled in a pre-war conspiracy involving Newtypes (humans with E.S.P. capabilities, a signature of the Gundam franchise), a newly-formed earth government, and the GX-9900 Gundam X aboard their ship.
Gundam X is notable for both being the first post-apocalyptic Gundam title as well as the shortest Gundam television series; it suffered low rating throughout it's broadcast and after being moved to a 6:00AM Saturday timeslot, rating dropped to the point that the series was cut to 39 episodes (down from it's originally planned 49 episode run).
The following firearms are seen in the anime series After War Gundam X:
Garrod Ran, the pilot of the GX-9900 Gundam X, carries a fictional semiautomatic pistol with him whenever he's not piloting a mobile suit. The pistol is a fairly conventional design and can best be described as a Double-action only handgun (with an external hammer and no manual safety) that has the slide of an M1911A1 and the frame of a Smith & Wesson 659.
Jamil's Pistol
Jamil Neate, captain of the Freeden and the first pilot of the Gundam GX-9900, carries a fictional semi-automatic pistol with him at all times. While it is a fictional design, it is based heavily on the CZ 75 series of pistols with changes to the triggerguard, safety, and grip panels. This weapon is also seen in the hands of Estardian rebels, and may be a pre-war military design.
Unknown Beretta pistol
When Garrod decides to auction off his newly acquired GX-9900 Gundam X, one of the bidders starts to draw a Beretta pistol (the exact model cannot be determined, as the only clearly-shown feature is Beretta's signature cutaway slide) before Garrod informs the audience that the cockpit is rigged with explosives.
Python
Witz Sou carries a 6-inch Colt Python with him while off duty.
Walther P5
Ennil El is seen with a Walther P5 pistol in episode 7.
Most of the New U.N.E.'s personnel are seen carrying a fictional Glock-like double-action only plastic-framed pistol. This weapon also occasionally shows up in the hands of forces outside the Post-War U.N.E.
Compact N.U.N.E. Pistol
Carris Nautilus is seen with a compact variant of the N.U.N.E.'s pistol.
Submachine Guns
N.U.N.E. SMG (Futuristic MP5K)
Most of the N.U.N.E. soldiers stationed on military bases or vessels are armed with a futuristic submachine gun. Upon closer inspection, this weapon is revealed to be just a Heckler & Koch MP5K with a number of cosmetic modifications, most notably a shell around the receiver, barrel and handguard. The crew of the Freeden also use these weapons during their prison break at the N.U.N.E. facility on Zonder Epta island.
Long-Barreled N.U.N.E. SMG
A variant of the N.U.N.E.'s SMG with a slightly longer barrel and barrel shroud is briefly seen near the end of the series.
MAC-11
Katokk Alzamille and his men infiltrate the Freeden armed with several MAC-11 submachine guns.
Fictional SMG
Estardian rebel Yurina Sonohara keeps a fictional submachine gun (based heavily on the Walther MP series in a box under her bed.
Rifles
Heckler & Koch G3A3
Numerous military and paramilitary groups, including the crew of the Freeden, are seen with H&K G3A3 battle rifles.
AK-47
A number of AK pattern rifles are seen in the hands of various smaller military forces, most notably the Republic of Estardia. The weapons are drawn rather generic, making it impossible to determine which exact AK variant they are intended to be.
Colonial Rifle
Several armed groups in the space colonies are seen using a fictional assault rifle. The rifle is shown to be able to function both in zero-gravity and in a vacuum, making it ideal for troops operating in and around the space colonies.
M21 Sniper Rifle
The M21 Sniper Rifle is used by several N.U.N.E. soldiers guarding the hangar where the newly-completed GX-9901-DX Gundam Double X is housed. All of these rifles are equipped with large night-vision scopes.
Heavy Weapons
Fictional Missile Launcher
During their escape from Zonder Epta island, several members of the Freeden use a fictional wide-bore missile launcher.
Fictional Long Missile Launcher
A second fictional RPG design is seen in the hands of the N.U.N.E. soldiers who try to stop Garrod from stealing the GX-9901-DX.
N.U.N.E. Heavy Machine Gun
Most N.U.N.E. A.P.C.s are armed with a swivel turret equipped with a (fictional) belt-fed heavy machine gun.
Futuristic M242 Bushmaster chaingun
Several N.U.N.E. armored vehicles are armed with futuristic versions of the M242 Bushmaster chaingun.
Other Weapons
Flare Pistol
Jamil is briefly seen using a flare pistol (or possibly a flare-launching attachment for a standard handgun) to communicate orders to the GX-9900.