Carbine Rifle

The Armalite/Colt M16 series of assault rifles are a staple of the GTA series, first appearing in GTA III, and then in every GTA game since. The M16 is useful in almost any situation, boasting accuracy, power and magazine capacity. Since the rifle fires the relatively powerful 5.56mm NATO round, it has noticeable recoil, albeit not nearly as much as the bigger AK-47. Throughout the games, the models of M16 derivatives features differ slightly, along with zoom capability, rate of fire, and power.

In Grand Theft Auto III, the firearm is an 'exaggerated' M16A1, with an extremely high rate of fire and a 60-round magazine. In GTA Vice City, the weapon is rendered more realistically — it resembles a Colt Model 733, although it is referred to incorrectly as the "M4".

Grand Theft Auto: San Andreas, and Liberty City Stories share the same 733. In Grand Theft Auto: Vice City Stories an M16A1 is used to maintain chronological accuracy, as the M4 was not introduced until 1994.

Variations
In chronological order:

GTA Vice City Stories (1984): An M16A1, named "Assault Rifle" in-game. It is correct in the model type as the M16A1, since the M16A2 entered service first with the United States Marine Corps in 1986, two years after VCS takes place. However, it is not totally chronologically accurate as 30-round magazines were introduced in the mid 70's, and its rate of fire is true to real-life M16s but the magazines are out of date. Lance Vance mentions the M4 during a mission, which is an anachronism.

GTA Vice City (1986): Modeled after a Colt Model 733, incorrectly titled "M4" within the game (the M4 was not introduced until 1994). It appears to have a Crane Stock instead of the original retracting stock. Originally, an M16A2 was meant to be in the game but it was later replaced by the 733. It's model can still be found in the game's files and it can be seen in some cutscenes (more notably - one of the deal ambushers during the intro cutscene is wielding one).

GTA San Andreas (1992): Reused Colt Model 733, again titled "M4" in-game. This version has an unrealistic reload animation wherein CJ charges the weapon with his left hand — real M733s are charged with a rear-mounted handle. This rendition has a 50-round magazine, not possible with real-life M733s, although they can use various STANAG magazines. In the mission Stowaway, a government agent is shown holding a camouflaged Colt M16A1; this, however, is never available to use in-game.

GTA Liberty City Stories (1998): Reused Colt Model 733, once again titled "M4" in-game, which is chronologically accurate. Has a realistic rate of fire, unlike its GTA III counterpart, but, like the San Andreas rendition, the reload animation is inaccurate.

GTA III (2001): Resembling the similar Armalite AR10, it is referred to as an M16. This version is a highly inaccurate and exaggerated weapon, with a 60-round magazine and an exceptionally high rate of fire. It is also missing the delta ring and is very boxy due to its low poly-count.

GTA IV (2008): An M4A1, titled "Carbine Rifle" in-game. This version is accurate both in timeline and overall features, although it sports a rear drum sight and lacks a carry handle, giving it some resemblance to the similarly-designed Heckler and Koch HK416. It is the longest-ranged non-sniper weapon in the game. It has "Property of Liberty City Arms" stamped where the magazine release is normally found, probably a reference to the "Property of U.S. Govt" stamped on its real-life counterpart. The bottom of the magazine is also stamped with the words "Stop Chillin Start Killin"

GTA Chinatown Wars (2009): Referred to as the "Carbine Rifle" in-game, it is difficult to identify this gun, however it has a full stock and long barrel, and its triangular hand guard on the HUD suggests a Colt M16A1. It has tremendous power, more akin to that of a machine gun, and it has a 50 round magazine like its San Andreas incarnation.

GTA III

 * Rockford, Staunton Island - Available to buy from Phil Cassidy's Army Surplus after completion of Arms Shortage ($5,000).
 * Bedford Point, Staunton Island - In the walled off area in the southeast part of Liberty Tree Offices (use the nearby Moonbeam, park it by the lowest part of the wall, climb the Moonbeam, and jump over the wall to access).
 * Cedar Grove, Shoreside Vale - On the back porch of the pink house (fourth from the west) at The Homes.
 * Cochrane Dam, Shoreside Vale - Near the top of the stairs for the west dam platform (not the top of the Cochrane Dam power house) at the base of the Cochrane Dam.
 * Delivered to Safehouses after collecting 90 Hidden Packages.

GTA Vice City

 * Prawn Island - In Studio B (behind the moon lander) at the InterGlobal Films complex.
 * Starfish Island - In the small room at the bottom of the Mansion's stairwell east of the main entrance after the completion of Rub Out.
 * Downtown - Available to buy from Ammu-Nation after completion of Rub Out ($5,000).
 * Little Haiti - In the middle of a cluster of shacks in the center of the North Haitian Neighborhood (just south of Auntie Poulet's house).
 * Little Havana - At the west side of the red and white building directly across the street west of Cafe Robina.

GTA San Andreas

 * Los Santos International Airport, Los Santos - Next to the runway, there are three white and yellow striped "ramps". Between those which are placed at the end of the runway.
 * Palisades, San Fierro - Behind a house next to the beach.
 * Area 69, Bone County - Behind a staircase leading to a control tower.
 * Pilgrim, Las Venturas - On the roof of the hotel.
 * The Four Dragons Casino, Las Venturas - In front of the casino if all 50 Horseshoes are collected.

GTA Liberty City Stories

 * Newport, Staunton Island - Among the containers at the docks and east of an unique stunt jump. It's located between the containers that are placed towards the Big Shot Casino.
 * Bedford Point, Staunton Island - Available to purchase from Phil Cassidy's Fully Cocked Gun Shop for $5,400 once the player beats the mission "Friggin' the Riggin'".

GTA Vice City Stories

 * Vice Point - Behind the building next to northernmost empire site.
 * Vice Point Mall, Vice Point - Available to buy from Ammu-Nation for $5,400 but the price is decreased to $4,050 when the player beats Phil's Shooting Range.

Grand Theft Auto IV
The M4A1 in Grand Theft Auto IV is referenced in-game as the Carbine Rifle. It can be purchased from the back-alley gun shops for $5,000 and each additional magazine will cost $100. It can be purchased from Little Jacob for $3,500 and each additional magazine costs $70. The Carbine Rifle can also be acquired from NOOSE officers and certain LCPD officers. It cannot be fired from cars, unfortunately. This weapon can kill with two headshots in multiplayer and single player, and is highly effective during Team Deathmatch. The M4A1 is the best weapon to have with its unique and perfect combination of capacity (with 30 round magazine, enough for 15 kills per magazine with perfect aim), power (2 headshots kill anybody without armor), range (most range on a non-sniper weapon in the game), and accuracy (resets to point aimed at almost immediately). This weapon can turn an early deficit into a comeback and even a blowout against stronger, better armed, or teams with more people, and is even more effective on single player and against police and pedestrians on multiplayer, where it is usually a 1-hit kill.

Bohan Algonquin Alderney
 * Bohan Industrial - Can be found in one of the offices inside the abandoned warehouse where the player rescues Roman from the mission Hostile Negotiation.
 * Algonquin Bridge - Can be found atop the roof of the pedestrian walkway on the Algonquin side, from the monorail station entrance.
 * Easton - Can be found well-hidden behind some trash cans on the east side of Grand Easton Terminal. The trash cans can be reached quickly through an alley on Jade St.
 * Leftwood - Can be found underneath the pier where the strip mall is situated off Beaverhead Ave. The strip mall can be seen from the cliff where Niko executes Aiden O'Malley during the mission Tunnel of Death.
 * Tudor - Can be found in a small loading dock area on the north side of the old Sprunk factory.
 * Tudor - Can be found atop the northern superstructure of the old rail bridge that spans the river which divides Tudor and the Acter Industrial Park.