Rifle

In the actual sense of the word, a "rifle" is a weapon with a spiral grooved barrel that imparts a spin on a projectile to obtain better levels of accuracy than a smooth-bore weapon, such as a musket or a shotgun. In the Halo universe sense of the word, a rifle can have several different meanings.

Summary
When regarding human weapons, there are a few forms of the word "rifle" used. An "assault rifle" is a kind of firearm, that may or may not be fully-automatic (and may posses a fire selector), that fires a round between a pistol or battle rifle. A "battle rifle" fires a large full-size rifle round and is usually, but not always, semi-automatic and often has a long full-size barrel. A "sniper rifle" fires a multitude of ammunition, almost exclusively in semi-automatic, in the goal of achieving high levels of accuracy, and typically longer ranges than assault or battle rifles.

Regarding Covenant weapons, a "rifle" is used not to describe the technical of the weapon itself, but in the role it plays on the battlefield. For example, the Plasma Pistol and Plasma Rifle, while similar, are used in two different ways. Basically, a rifle is larger than a pistol, often requiring two hands to use (except while dual-wielding), but is smaller than a heavy weapon like a Brute Shot or Fuel Rod gun. This applies to the Plasma Rifle, Brute Plasma Rifle and Beam Rifle. The Covenant Carbine is also considered a rifle and the Type-25 Carbine (commonly called a Spiker or a "Spike Rifle") can fit into this list as well.

There are a total of 24 known appearances of a "rifle" in the Halo universe

Assault Rifles
Assault Rifles are are military service weapons that that fires intermediate rifle cartridge; merging the light machine guns' firepower with the submachine guns compact design.

MA series

 * MA37 Individual Combat Weapon System - The Precursor to most variants of the MA series.
 * MA2B Individual Combat Weapon System - A stripped-down variant MA5B assault rifle.
 * MA3A Individual Combat Weapon System - An earlier version of the MA-series assault rifles.
 * MA5B Individual Combat Weapon System - The most common MA-series rifle, featured in Halo: Combat Evolved.
 * MA5C Individual Combat Weapon System - Newer version of the MA5B rifle, featured in Halo 3.
 * MA5K Carbine - Uncommon MA-series rifle wielded by special operations groups such as the SPARTAN-IIIs.

Battle Rifles
Battle Rifles are military service weapons that fires a full power rifle cartridge; providing greater effective range, penetration, and stopping power than assault rifles.
 * XBR55 Battle Rifle - Halo: Contact Harvest-prototype Battle Rifle.
 * BR55 Battle Rifle - Two-handed firearm firing in 3-round bursts featured in Halo 2.
 * BR55HB SR Battle Rifle - An updated version of the BR55 for Halo 3.

Marksman Rifles
Marksman Rifles are military service weapons that fires a anti-personnel/magnum-caliber rifle cartridge; used to fill the "marksmanship gap" between the battle rifle and the sniper rifle.
 * M392 Designated Marksman Rifle

Nonlinear Rifles

 * M6 Grindell/Galilean Nonlinear Rifle - A shoulder-carried, anti-vehicular, directed laser weapon. Only featured in Halo 3, Halo Wars, Halo 3: ODST and Halo Reach. It is better known as the Spartan Laser.

Sniper Rifles
Sniper Rifles are military service weapons that fires anti-material/anti-vehicular rifle cartridge; used for a more accurate and long range shots than other rifles or small arms.
 * Sniper Rifle System 99 Anti-Matériel - Variant of the SRS99C Sniper Rifle used in Halo: Reach.
 * Sniper Rifle System 99C-S2 Anti-Matériel - Long-range two-handed projectile weapon featured in Halo 2.
 * Sniper Rifle System 99C-S2 Anti-Matériel - Variant seen in Halo: Ghosts of Onyx.
 * Sniper Rifle System 99D-S2 Anti-Matériel - A variant of the SRS99C-S2 AM Sniper Rifle used in Halo 3.
 * M99 Special Application Scoped Rifle - A Gauss Sniper Rifle, used by Avery Junior Johnson on Operation: KALEIDOSCOPE

Projectile

 * Type-33 Carbine - A one-handed Needler. Used in a similar fashion to the Assault Rifle.
 * Type-31 Rifle - A cousin of the Needler. Used in a similar fashion to the Marksman Rifle. More commonly known as the Needler Rifle.
 * Type-25 Carbine A one-handed "spike rifle". Used in a similar fashion to the Submachine Gun. More commonly known as the Brute Spiker.
 * Type-51 Carbine A very scaled down version of the Fuel Rod Gun. Used in a similar fashion to the Battle Rifle. More commonly known as the Covenant Carbine.

Particle

 * Type-50 Sniper Rifle System - Long-range two-handed weapon, primarily used by Jackal Snipers. More commonly known known as the Beam Rifle.

Plasma

 * Type-25 Directed Energy Rifle - One-handed plasma-operated weapon typically wielded by Sangheili. More commonly known as the Plasma Rifle.
 * Type-25 Directed Energy Rifle/Jiralhanae variant - Variant of the standard Plasma Rifle wielded by Brutes. Seen only in Halo 2 and Halo 3: ODST.
 * Type-51 Directed Energy Rifle/Improved - Two-handed plasma-operated weapon used by Sangheili in particular. More commonly known as the Plasma Repeater.
 * Type-52 Special Applications Rifle - A sniper-style weapon that fires a continuous beam of magnetically contained plasma. More commonly known as the Focus Rifle.

Other

 * Gravity Rifle - A Halo: Combat Evolved deleted Covenant long range rifle.