Support weapon

Support weapons are a class of heavy weapons.

These weapons are much larger than the standard handheld weapons, and require two hands to heft. Consequentially, players move more slowly while holding them, and cannot throw grenades or melee opponents while holding the weapons, thus they become very vulnerable to enemy grenades and vehicles, since they almost can't be dodged.

With the exception of the flamethrower, which is described as being usually carried about on a cart, support weapons are generally weapons that were originally designed to be fired from stationary mounts, but have been ripped off of their bases and carried around. Thus while they have limitless ammunition while in their mounts, once removed and carried the ammo they have becomes limited. Also, the accuracy of the turret can become more of a problem, being handheld of course.

List of support weapons

AIE-486H Heavy Machine Gun - the machine gun turret featured in Halo 3 and Halo 3: ODST.
LAU-65D/SGM-151 - otherwise known as the Missile Pod, featured in Halo 3 and Halo 3: ODST.
M7057/Defoliant Projector - otherwise known as the Flamethrower, featured in Halo PC, Halo 3, and Halo 3: ODST.
M247 General Purpose Machine Gun - the machine gun turret featured in Halo 2.
M247H Heavy Machine Gun - the machine gun turret featured in Halo: Reach and Halo 4.
Type-42 Directed Energy Support Weapon- otherwise known as the Plasma Cannon, featured in Halo 2.
Type-52 Directed Energy Support Weapon- otherwise known as the Plasma Cannon, featured in Halo 3, Halo 3: ODST, Halo: Reach, and Halo 4.

Trivia

In Halo PC, Flood combat forms have the animation and programing required to wield the Flamethrower. However, when they shoot it at a player directly in front of them or below them, they point it at a 45 degree angle in the air, making them not so effective when wielding it.
In Halo 3, If the Arbiter is killed near a support weapon that is off its mount, he will use it when he gets back up.
Other than the Flamethrower, all deployed support weapons in multiplayer have limited ammo regardless of whether the infinite ammo option is turned on or off.
Standard weapons like the Brute Shot and the Sentinel beam function similarly to support weapons: they are held the same way, and have more firepower than normal weapons though the player can still melee or throw grenades and the player can only carry one other weapon while wielding them.
Support weapons are especially useful in multiplayer because the third-person point of view allows the wielder to see around corners without making them visible or vulnerable to enemy fire.
Shields and armor compensate somewhat for the limited mobility. As such, support weapons are more of a liability in SWAT or sniper games, as the gunner can easily be killed with a headshot.

Gallery

Missile Pod.jpg

The Missile Pod
A MJOLNIR Mk.VI/EVA wielding a Missile Pod in Halo 3.
The Heavy Machine Gun.
MJOLNIR Mk.VI Powered Assault Armor (CQB).jpg

A MJOLNIR Mk. VI/C wielding a Heavy Machine Gun in Halo 3.
The M7057 Defoliant Projector.
A SPARTAN wielding a Flamethrower in Halo 3.
The Plasma Cannon.