Assault Rifle
The Assault Rifle has been brought back from the first installment in the Halo series with a few modifications like greater accuracy, more range, and more damage per hit. Accuracy is high when fired in bursts, and the overall accuracy of this weapon is good at medium ranges (even higher at close). This is a very good all round weapon that can be effective in almost all situations if used correctly, and is an even better weapon with grenades, and melee attacks (this goes with any weapon).
Battle Rifle
A semi-automatic rifle that fires in three round bursts, the Battle Rifle, or BR as it is commonly referred to as, is a favorite among experienced and skilled players becuase it can kill an enemy with a quick four headshots. The BR is a very common weapon, and if used well can be one of the best in a game. Originally found in Halo 2, the BR has not seen many changes, but seems to be much more widely used in Halo 3. As they say, you're in a battle. Bring your Battle Rifle.
Beam Rifle
The Covenant beam rifle is the counterpart to the human Sniper Rifle. Allowing use of 5x and 10x magnification scopes, the beam rifle carries eleven shots on its standard battery. While not as common as its human counterpart, the beam rifle fires in a pink blast, and is more silent and stealthy.
Brute Shot
The Brute Shot has been changed, going from a hard-to-find but effective weapon to a mainstay of campaign and a solid choice in multiplayer. It fires grenades which explode on contact, doing serious damage, especially in confined spaces.
Covenant Carbine
Unlike the Battle Rifle, the Carbine has seen few changes since Halo 2. It remains a powerful and accurate semiautomatic weapon with a moderately-sized clip. Because of the changes made to the Battle Rifle however, the Carbine is now far more accurate than its human counterpart, and is rapidly overcoming the second-class status it had in Halo 2.
Energy Sword
The energy sword is one of the most deadly close range weapons in the game. Back from Halo 2, it now has a battery that is used up after ten kills, which makes strategy and proper use of this weapon even more important.
Firebomb
The Type 3 AP/AM grenade is a Brute device that breaks when thrown, releasing a flaming gel that sticks to hard surfaces. The gel burns for around four seconds and can be used as a barrier or to kill players directly.
Flamethrower
The flamethrower is a support weapon, strong against both vehicles and ground units, but balanced by a relatively short range and limited ammunition.
Frag Grenade
Frag grenades have seen a drastic power nerf to make it fit more into the balance of the game. It can be devastating to infantry if used correctly. A well timed frag can also tip a Warthog.
Fuel Rod Gun
A heavy Covenant weapon that is a counter to the Rocket Launcher. Before Halo 3 it was only available for multiplayer use in Halo PC.
Gravity Hammer
A weapon similar to that of Tartarus' in Halo 2. A melee weapon that has a lunge shorter than the energy sword but has the power to knock back projectiles or vehicles out of the way.
Machine Gun Turret
Remember being sniped in Halo 2 when you were using the turret? This time round you can pick up the turret and move position! So no more easy target for snipers. Not only can you walk with this thing but its more powerful and the aim too has improved. Go take out their whole team or that one annoying Mongoose!
Magnum
The M6G Magnum is a dual-wieldable semiautomatic pistol that does great damage to shields and flesh, has a headshot bonus, and is accurate out to medium range. It resembles Halo's M6D Magnum, but lacks a scope and does slightly less damage.
Mauler
A dual wieldable Brute shotgun that has less power than the human varient when single wielded, but can be more powerful when two are wielded.
Missile Pod
One of the new support weapons introduced in Halo 3. Like all support weapons, it slows the player and forces a third-person perspective. The missile pod can be thought of as a rebalanced answer to Halo 2's tracking rockets: it has a larger ammunition capacity than the rocket launcher and is capable of tracking vehicles, but each missile does less direct and splash damage than a rocket.
Needler
One of Halo 3's more unique weapons, the needler is a mid-range weapon that fires pink needles that track enemies well at close to mid ranges. While it cannot be dual wielded like in Halo 2, it is now more powerful as a single wielded weapon.
Plasma Cannon
The Plasma Cannon, like it's Human counterpart, the Machine Gun, is a 3rd person Support Weapon that can be pulled off from the covenant turrets on a level.
Plasma Grenade
A Grenade of Covenant origin. That sticks to enemies and vehicles. It has seen a slight nerf in general, like the Frag Grenade. But still can be devastating if used correctly.
Plasma Pistol
The Plasma Pistol is the most basic of the Covenant weapons. The non-charged shot has little use. However, the overcharge can completely deplete enemy Shields allowing an easier kill with other weapons or it can overheat a vehicles engine, not allowing it to move for a few seconds. Pistol Whips! However it still allows the turret on the Warthog to fire.
Plasma Rifle
A rifle of Covenant origins that fires plasma bolts at the enemy. Like most Covenant weapons that fire plasma it helps to deplete shields faster than Human weaponry.
Rocket Launcher
The rocket launcher oncemore has no vehicle lock-on. That attribute was lost to the Missile Pod. In the Beta, the rocket had slight auto aim, it would bend course very slightly towards a player.
Sentinel Beam
A laser or Forerunner origin. The weapons of the sentinels. The Sentinel Beam is a battery powered laser.
Shotgun
The shotgun now sees a slight reduced clip. But is still devastating at close range.
SMG
An old Halo 2 favorite, the SMG is no longer given the role of starting weapon. However, it is still useful at close to mid ranges, can be dual wielded, and is considered a counterpart to the Brute Spiker and the covenant plasma rifle.
Sniper Rifle
Long Range Sniper rifle with 5-10x scope. The Sniper Rifle can kill faster than Any other weapon in the game, due to its Single head-shot kill ability. It is the most inferentially accurate weapon in the Halo series. It's Covenant counterpart is the Beam Rifle. It is rare to find multiple Sniper Rifles on maps. It is favorite to most long-range shooters. It is a VERY powerful weapon (boasting single head-shot kills and two shot kills on the rest of the body) It now has a Night-vision scope (on the scope display when not scoped-in). The Sniper Rifle is VERY effective at long ranges, but can also be effective at Medium ranges, or Close ranges if used correctly.
Spartan Laser
Portable Power! The Halo 3 Spartan Laser is one of the new members of the Halo armory. It is designed for anti-vehicle use, but is also very effective as an anti-personnel weapon. The directed energy mechanism fires a highly charged laser beam that, if it hits, totally destroys the target. A brief charge time before firing and a small laser "pointer" coupled with humming noise makes surprise attack with this weapon somewhat tricky, but players caught out in the open against the Spartan Laser are usually left counting down to their respawn.
Spike Grenade
The Spike Grenade, a Brute technology, is one of the two new grenade types introduced in Halo 3. Spike grenades can be stuck to nearly any surface, including players, and spray shrapnel in one direction when they explode. Like a Plasma Grenade, sticking an opponent with a spike grenade is fatal.
Spiker
What some people call it the brute SMG. It shoots out the spikes like the Spike Grenade.