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 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 weapon. 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 20 known appearances of a "rifle" in the Halo universe:

MA series

 * MA37 Assault Rifle - The Precursor to most variants of the MA series.
 * MA2B Assault Rifle - A stripped-down variant MA5B assault rifle.
 * MA3 Assault Rifle - An earlier version of the MA-series assault rifles.
 * MA5B Assault Rifle - The most common MA-series rifle, featured in Halo: Combat Evolved.
 * MA5C Assault Rifle - Newer version of the MA5B rifle, featured in Halo 3.
 * MA5K Assault Rifle - Uncommon MA-series rifle wielded by special operations groups such as the SPARTAN-IIIs.

Battle Rifles

 * XBR-55 - 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.

Nonlinear Rifles

 * Anti-Vehicle Model 6 Grindell/Galilean Nonlinear Rifle (W/AV M6 G/GNR) - A shoulder-carried, anti-vehicular, directed laser weapon. Only featured in Halo 3, Halo Wars and Halo 3: ODST, it is better known as the Spartan Laser.

Sniper Rifles
Sniper Rifles are used for a more accurate and long range shots than other small arms
 * SRS99C-S2 AM Sniper Rifle - Long-range two-handed projectile weapon.
 * SRS99C-S2 AMB Sniper Rifle - Seen in Halo: Ghosts of Onyx.
 * SRS99D-S2 AM Sniper Rifle - Halo 3-variant of the SRS99C-S2 AM Sniper Rifle.

Projectile

 * Type-51 Carbine A very scaled down version of the Fuel Rod Gun it has similar damage to the Battle Rifle
 * Type-25 Carbine A One-handed "spike rifle" better known as the Spiker.

Particle

 * Type-50 Sniper Rifle System - Long-range two-handed weapon, primarily used by Sniper Jackals.

Plasma

 * Type-25 Directed Energy Rifle- One-handed plasma-operated weapon typically wielded by Sangheili.
 * Jiralhanae Type-25 Directed Energy Rifle- Variant of the standard Plasma Rifle wielded by Brutes. Seen only in Halo 2 and Halo 3: ODST.

Other

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