SPAS-12: Since it is used in semi-automatic 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 (though this mistake isn't seen in Liberty City Stories).
AK-47: The reloading animation erroneously depicts the charging handle as being on the left side instead of the right.
M4: It is incorrectly cocked from left side instead of the rear. Furthermore, the "M4" in Vice City, San Andreas and Liberty City Stories is actually modeled after the shorter-barreled Colt Model 733, and the front sight is oddly modeled-on backwards.
Sniper Rifle: Incorrectly needs only to be bolt cycled, and never seen being reloaded.
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 extremely impractical in reality. Not to mention that the heavy power supply that would allow it to fire isn't seen in-game, and that it would make it unable to be freely carried during gunfights like a traditional firearm.
Pistol: Same reasoning as in Vice City regarding 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.
Sawn-off Shotgun: An empty shell is incorrectly shown being ejected after each shot, which shouldn't be the case on such a double-barreled shotgun.
Micro SMG: The HUD icon shows a MAC-10 instead of a Micro Uzi.
MP5: Same reasoning as in Vice City regarding the HK94A3.
AK-47: Despite being referred to as such in-game, it is actually modeled after the Chinese Norinco Type 56 clone.
M4: Same reasoning as in Vice City. Additionally, the magazine model is that of the 30-round capacity, whereas a 50-round magazine for it would either be longer or a drum magazine.
Rifle: Incorrectly needs only to be lever cycled, and never seen being reloaded.
Sniper Rifle: The HUD icon shows a PSG-1 instead of a Remington 700.
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
MP5: It appears to be the SP89 variant (converted to full-auto), which is anachronistic to the game, since the weapon was made in 1989; the MP5K (or simply the standard MP5) would be more appropriate.
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, whereas the weapon in-game has a 30-round capacity.
Sniper Rifle: It 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 has a custom pistol grip, which is not the case for the real weapon.
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 right-mounted 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.
Sniper Rifle: The bolt is erroneously operated from the left side instead of the right, despite the model clearly showing the bolt on the right side. Furthermore, the model shows a 10-round magazine, while the weapon in-game only holds 5 rounds.
Grenade Launcher in The Lost and Damned and The Ballad of Gay Tony: The grenades' behavior of exploding on impact only on enemies/vehicles is unrealistic; obviously they should detonate on impact on any surface.
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 instead of a submachine gun, and this weapon was not produced in reality, as the project has 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.
Grenade Launcher: Same reasoning as in in The Lost and Damned and The Ballad of Gay Tony.
Rail Gun: Similarly, real rail guns are only mounted; portable ones don't exist.
Magazine modeling errors
The AP Pistol, Pistol .50, SNS Pistol and Vintage Pistol have long projecting magazine models when the extended magazine attachment is used, which is not possible on their real counterparts.
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).
MG: The standard magazine model shows a circular drum, looking similar to that of the RPD); such a drum magazine cannot attach to the socket under the PKM.
AK-47: The 40-round model seen with the extended magazine attachment is not enough to hold the in-game 60-round capacity.
Grenade Launcher: The model has 6 tubes in its cylinder (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.
M16: 20 rounds (since it mostly looks like an Armalite AR-10; either way the 60-round capacity for an actual M16 wouldn't fit in the game's period since 60-round AR-15 magazines weren't available in 2001)