Pistol: Incorrectly referred to as "nine" by some characters in-game; the real M1911A1 is chambered in .45 ACP.
Pump Shotgun: Incorrectly needs only to be pumped, and never seen being reloaded.
AK-47: Despite being referred to as such in-game, it is actually modeled after the Romanian WASR-2 clone. Furthermore, the reloading animation erroneously depicts the charging handle as being on the left side instead of the right.
M16: Despite being referred to as such in-game, it is actually modeled after the ArmaLite AR-10. Furthermore, it is erroneously cocked from left side instead of the rear.
Sniper Rifle: Incorrectly needs only to be bolt cycled, and never seen being reloaded.
Rocket Launcher: Being an M72 LAW in-game, it is incorrectly treated as being reloadable; the real one can only be used once.
SPAS-12: Since it is used in semi-auto mode, it should be cocked when reloading by using the charging handle at the right side of the weapon, not the pumping handle (which would be locked if the SPAS-12 is used in semi-auto). Furthermore, for some reason, it erroneously seen being pumped after firing (when interrupting a firing process).
MAC-10: The HUD icon shows a Micro Uzi instead of a MAC-10.
MP5: Despite being referred to as "MP" in-game, it looks more like the civilian HK94A3 variant converted to full-auto fire.
Ruger: The reloading animation erroneously depicts the charging handle as being on the left side instead of the right.
M4: Despite being referred to as such in-game, it is actually a Colt Model 733. While the Model 733's presence is accurate in the game, an actual M4 would be anachronistic, since this variant was produced in 1994 (although early M4 prototypes existed during the game's period). Moreover, the front sight is modeled-on backwards, and the weapon is erroneously cocked from left side instead of the rear.
Laser Sniper: The weapon is incorrectly labeled as ".308 Sniper" in most versions of the game; the real SR9(T) is chambered in 7.62x51mm. Furthermore, it is anachronistic to the game, since the weapon was introduced in 1990; other variants like the PSG-1 or the MSG90 would be more appropriate.
M60: The reloading animation erroneously depicts the charging handle as being on the left side instead of the right.
Minigun: Contrary to what many movies and video games suggest, using a man-portable/hand-held minigun is impossible in reality.
Pistol: Same reasoning as in Vice City concerning the magazine model, and the weapon is incorrectly referred to as "9mm", whereas the real M1911A1 is chambered in .45 ACP. Furthermore, the HUD icon shows a Glock 17 instead.
Silenced Pistol: Same erroneous caliber and magazine model as the non-silenced variant, as well as the HUD icon showing a silenced Glock 17 instead of an M1911A1.
Pistol: Being a Beretta with a 17-round magazine, it should have have a deeper base than the one on the 15-round model in the game (furthermore, the Beretta's 17-round magazines were not in existence during the game's setting).
The stainless steel version seen in the intro cutscene of "Nice Package" is anachronistic, since the Inox variant of the Beretta 92FS was introduced in 1990
M16: Erroneously referred to as "M4" by Lance Vance (which would be anachronistic), and still cocked from the left instead of the rear. Furthermore, it is modeled with a 20-round magazine, while the weapon in-game has a 30-round capacity.
Sniper Rifle: Incorrectly operates in bolt-action mode; the sniper rifle in-game is modeled after the M21, which is semi-automatic. Furthermore, the HUD icon shows a Remington 700 instead of an M21.
Laser Sniper: The HUD icon shows a PSG-1 instead of an SVD Dragunov.
Rocket Launcher: Like in GTA III, being an M72 LAW in-game, it is incorrectly treated as being reloadable. Furthermore, it erroneously has a pistol grip.
Minigun: Same reasoning as in Liberty City Stories.
Pistol: It is incorrectly referred to as "9mm", whereas the real Glock 22 is chambered in .40 S&W. Additionally, a cutscene and dialogue in The Ballad of Gay Tony indicates that the weapon has a trigger safety, whereas the real Glock 22 doesn't have one.
Pump Shotgun: The in-game model shows an airsoft version rather than an actual Ithaca 37 Stakeout, and the magazine tube is not long enough to have a high ammo capacity like the weapon in-game. Furthermore, the reloading animation always depicts two shells being inserted, regardless of the number of shells needed
Combat Shotgun: When blind-firing, the weapon incorrectly fires in pump-action mode instead of semi-auto. Furthermore, the reload always shows two shells being inserted, and the weapon is pumped (the real one not having a pumping handle) instead of being cocked with the charging handle.
Assault Shotgun in The Lost and Damned: It is incorrectly fully-automatic in-game and pump-action when blind-firing, whereas the real Street Sweeper is semi-automatic. When standing, it is erroneously reloaded only by cocking it, and this is incorrectly done from left side instead of the right. Furthermore, when crouched, the reloading animation depicts the weapon as having a detachable magazine, whereas the real one requires shells to be loaded one-by-one into the fixed magazine.
Automatic Shotgun in The Ballad of Gay Tony: It is incorrectly semi-automatic in-game and pump-action when blind-firing, whereas the real AA-12 is fully-automatic (although in-game the variant with explosive shells is used in full-auto mode if the player is falling or being burned).
SMG: When reloading, the charging handle incorrectly locks back on its own before changing magazines. Furthermore, when crouched, the weapon is erroneously cocked one more useless time after reloading and releasing the bolt.
AK-47: When standing, it is erroneously reloaded only by cocking it without replacing magazines, and like in previous games it is incorrectly cocked from left side instead of the right.
Carbine Rifle: When standing, it is erroneously reloaded only by cocking it, and this is incorrectly done from left side instead of the rear.
Sniper Rifle: The bolt is erroneously operated from the left side instead of the right.
Assault Shotgun: Despite having the appearance of the pump-action UTS-15 with an fictionally added box magazine, it function like an AA-12 instead, due to the fully-automatic fire; it has a custom charging handle added to the right side of the receiver to replace the pump.
Bullpup Shotgun: In reality, the KSG's tube selector should be switched after firing half of the magazine capacity; otherwise it would only be able to fire this half. Furthermore, the reload only shows one shell being inserted.
Musket: Erroneously classified as a shotgun instead of a rifle. Furthermore, during the reload, the player doesn't push the ball with a ramrod, and neither do they load the pan with gunpowder nor re-cock the hammer.
Assault SMG: It is largely on based on the Magpul PDR, which is an assault rifle, and this weapon was not produced in reality due to the project having been abandoned.
Gusenberg Sweeper: It inaccurately shares ammo with the light machine guns instead of the submachine guns.
Heavy Sniper: When aiming the scope, the sound of a bolt operation can be heard after each shot, which is erroneous since the weapon is semi-automatic.
Rail Gun: Similarly, real rail guns are only mounted; portable ones don't exist.
Magazine modeling errors
All the handguns (except the Stun Gun) have long projecting magazine models when the extended magazine attachment is used, which is not possible on their real counterparts (except for the Pistol and the Heavy Pistol).
Heavy Pistol: Given the high ammo capacity for this M1911 variant, it should have a long projecting magazine (which is used of the "Extended Clip" model) instead of the single-stack one, and a drum mag when the extended magazine attachment is used.
Sawed-Off Shotgun: The magazine tube is too short to have a high ammo capacity like the weapon in-game.
MP5: Given the ammo capacity, it should have the 30-round magazine model by default (instead of the 15-rd model), and a drum mag when the extended magazine attachment is used (instead of the 30-rd model).
Gusenberg Sweeper: The standard 30-round version should have a normal box magazine (whereas the the drum one should only be used with extended magazine attachment).
AK-47: Given its capacity, the extended magazine model should be a drum magazine (instead of the 40-rd model seen in-game).
Grenade Launcher: The model has 6 tubes in its cylinder (which is the capacity of the real Milkor MGL), yet the weapon in-game erroneously has a capacity of 10 grenades.
AP Pistol: It is depicted with a traditional pistol slide instead of the real Colt SCAMP's design.
Pistol .50: The weapon model lacks the long beaver-tail of the actual Desert Eagle.
Advanced Rifle: It is modeled with an open pistol grip and a traditional trigger guard (similarly to the MTAR-21 variant), although early screenshots showed a more correct CTAR-21.
Special Carbine: It utilizes straight metal magazines similar to the Galil 7.62 (contrary to the actual G36C, which is chambered in 5.56). Additionally, it has a custom charging handle that juts out on both sides of the weapon, rather than a flush charging handle.