Carbine

Summary

The Type-51 Carbine, is the Covenant equivalent of the BR55HB SR Battle Rifle and the BR55 Battle Rifle. It is a semi-automatic weapon that combines high accuracy with decent damage power, used primarily for mid-range combat. Unlike most standard-issue Covenant weapons, such as the Plasma Rifle, the Carbine doesn't run on a battery. Instead, it fires solid, radioactive projectiles, similar to that of a Fuel Rod Gun, from an 18-round cylindrical magazine, sometimes suggested as being analogous to a "charger clip" style magazine, (such as in an M1 Garand or M91 Carcano used in WWII). It has a similar feature to the Assault Rifle and the Battle Rifle in that it tells how much ammunition is in the magazine. It is sometimes carried by Brutes and Elites, though this is not usually their preferred weapon, as they prefer more common weapons, such as the Spiker for the Brutes, and the Plasma Rifle and Energy Sword for the Elites. In the hands of a Brute, or Elite, they can be extremely deadly, allowing them to engage their enemies at mid range with deadly accuracy. This is the weapon of choice for Jackal Snipers in Halo 3, along with the Beam Rifle. In fact, it almost replaces the Beam Rifle for Sniper Jackals, unless on a higher difficulty.

Advantages

With a 2x zoom ability its possible to make deadly head shots from large distances. It is also effective at killing an unshielded opponent in multiplayer, and has equal effectiveness to an unshielded enemy as that of a Battle Rifle and the M6C, M6D and M6G pistols. Like those weapons, it will kill an unshielded opponent with one shot to the head instantly.

Because the Carbine fires shielded radioactive isotope rounds (not plasma bolts), it is more useful against the Flood than the other Covenant weapons, excluding explosives, as it can destroy the embedded Infection Form. One of the best uses for this weapon is to shoot the armored headpiece off of Brutes, after which another head shot from the weapon will dispatch them. The Carbine is also more effective at taking down Brute Chieftains than the Battle Rifle.

The Carbine, when used correctly, can take down an enemy slightly faster than a Battle Rifle can, though it requires more aim, taking at least seven shots to the head in Multiplayer. Damage done by the Carbine is slightly lower than that of the Battle Rifle (if all three bullets hit), but it fires about twice as fast making it comparably deadly.

The Covenant Carbine has one other advantage over the Battle Rifle. It is able to do damage to Jackal Point Defense Gauntlets, Enforcer shields, Stationary Shield Generators, Brute Power Armor, and other shields of such nature, though not as much as other Covenant weapons.

It's also very useful in the Halo 2 campaign against Brutes. The weapon can land head shots, and can be used at almost all ranges. Because Brutes have very little head protection, a skilled player can take out three or four Brutes with one magazine or with one shot to the head when playing on easier difficulties. Also since its a fast shooting weapon it is useful to shoot off a Brute's helmet and then shoot a Brute's head making a quick kill.

The magazine, though holding less ammo than the BR, will last longer because the BR fires in three shot bursts.

Disadvantages

The Carbine's only disadvantage is that your position is clearly shown by a yellowish-green trail with each shot, just like the Sniper Rifle's vapor trail or the Battle Rifle's tracer trail. Its sound may also attract the attention of nearby players. Also in Halo 3, a green mist is ejected from the ammo slot, giving away the player's position if reloaded. It also has weaker firepower, so it usually only gives an advantage to more skilled players, though less skilled players with some degree of luck may be able to use it effectively as well. Also, it is not as effective if you use rapid fire, but a sniper with good aim and that can take cover well can be deadly with a carbine.
edit Changes From Halo 2

In Halo 2, the Carbine's green semicircle will change into a red circle when the magazine is empty. It will not do this in Halo 3.

In Halo 2, when you zoom in, the view is a group of seven hexagons melded into a honeycomb shape. In Halo 3, four lines will separate from the center in addition to the hexagons, almost on the same plane as the reticule lines.

In Halo 3, the Carbine has a green patterned circle on top of the magazine indicator.

Carbine ammo is much more abundant, double the amount usually given in Halo 2.

Halo 3

During the Halo 3 Beta, there were concerns that the Carbine was overpowered compared to its counterpart the Battle Rifle. This disparity was attributed to an excessively high rate of fire, which allowed it to inflict more damage over time than the Battle Rifle did. In the final game, the Carbine's rate of fire has been reduced to about that of the Halo 2 version. Furthermore, its power has been reduced slightly from its Halo 2 counterpart. It now takes eight head shots to kill a fully-shielded opponent in multiplayer, as opposed to the shorter seven in Halo 2 and the Halo 3 Beta. In addition to the reduction in per-shot damage, the Carbine, also starts with two spare magazines when found in multiplayer, whereas it only started with a single spare magazine in Halo 2 multiplayer.

Also, in the Beta, when reloaded, it did not seem to emit a small cloud of gas as it does in the final version of the game.

As far as the Covenant Carbine's role in the campaign, it is still occasionally found to be used by Brutes, but it can most frequently be found in the hands of Jackals, who utilize it in a marksman role.