I suggest using the DMR and plasma pistol combination throughout the majority of the Halo Reach campaign because it allows you to easily eliminate most enemy types. However, every weapon has pros and cons, and you should experiment with all of them to choose a loadout that best fits you play style. The below descriptions are only intended to be used for firefight and campaign. The strategies for fighting AI do not apply for fighting human players and the uses of each weapon greatly differ in online matchmaking. The numbers after the name of each weapon indicate the bullets per magazine and maximum extra ammo.
UNSC weapons tend to be more accurate than their covenant equivalents. You can occasionally find UNSC guns from the corpses of UNSC soldiers and ODSTs, but your major supply will be from the areas of the level that they are placed in. Other than the grenade launcher and rocket launcher, all UNSC guns fire infinitely fast projectiles, so are impossible to dodge. UNSC guns inflict bullet damage, making them more effective against health and less effective against shields.
Assault rifle: 32/288: The assault rifle is an accurate fully automatic rifle with very little recoil. If you find that you are too poor of an aimer to achieve headshots with the DMR, I would suggest using the assault rifle instead. When using automatic guns like the assault rifle, aim for the body instead of the head. There is usually plenty of assault rifle ammo placed throughout each mission.
DMR: 15/60: The DMR is an incredibly accurate, semi-automatic rifle with a 3x zoom. A single headshot with this weapon imparts infinite damage on unshielded enemies without helmets. This makes it incredibly useful for killing every enemy type except Hunters. If you have a high accuracy, there is enough DMR ammo placed throughout each mission that you will not run out.
Grenade launcher: 1/15: The grenade launcher fires remote detonated grenades that inflict higher damage than frag grenades and inflict EMP damage. The EMP capabilities allow grenade launchers to both stun vehicles and remove shields. Some players prefer using grenade launchers to plasma pistols, yet grenade launcher require much more skill to use and do not fully remove the shields of some higher ranking Elites. Furthermore, grenade launchers are also rarely seen throughout the Halo Reach campaign so cannot be depended on for fighting Elites. They are, however, very useful when fighting Hunters.
Magnum: 8/40: The magnum is an accurate, semi-automatic pistol very similar to the DMR. The magnum also has the property of instant killing headshots. However, the magnum is less accurate and has only a 2x zoom, so the DMR is a far superior weapon. The magnum also inflicts less damage per shot, so cannot remove Brutes’ helmets efficiently. A magnum should be used in place of a DMR when a DMR is not present.
Rocket launcher: 2/8: Rocket launchers fire fast moving rockets, 17 wsm Ammo for sale dealing immense damage and high collateral damage. However, rocket launchers are incredibly rare, and should usually be saved for Hunter fights. A single rocket is capable of killing most infantry and heavily damaging a Hunter. Rocket launchers also have a 2x zoom and a homing capability on vehicles. To achieve a lock-on, follow the vehicle with your reticule until the reticule turns red and beeps.
Shotgun: 6/40: The shotgun inflicts large amounts of damage at close range, but quickly decreases in power the farther away you are from the target. Even at point blank range, a shotgun requires multiple shots to kill Elites and Brutes on Legendary, so is not very useful for this purpose. Shotguns also require you to charge the enemy, a process that usually results in death on Legendary. However, shotguns are very useful against Hunters, and are always found before a Hunter fight.
Sniper rifle: 4/20: Sniper rifles are the most accurate weapons in Halo Reach, containing a variable zoom of 5x and 10x. Like a DMR, a sniper rifle will instantly kill any enemy with an unprotected head. In addition, sniper rifles impart enough damage to instantly kill helmeted enemies without shields and enemies with low capacity shields with a single headshot, making sniper rifles useful for killing both Brutes and Elites from a distance. However, sniper rifles are very rarely found throughout the campaign.
Spartan laser: Energy weapon/25 units per shot: The Spartan laser is the only UNSC weapon that uses energy instead of ammunition. To fire a Spartan laser, hold down the trigger to charge it up, and it will fire after approximately one second. This laser is powerful enough to kill any infantry except a Hunter and most vehicles in a single shot. Two shots will allow you to destroy stronger vehicles including wraiths. Spartan lasers are incredibly rare, and since they must be charged up, they are difficult to use as well. Spartan lasers have a 2x zoom.
Target Locator: 1/2: The target locator provides the coordinates for an artillery strike. By holding the trigger, it paints a circle on the ground that an orbital artillery strike will fire upon. This is useful for destroying enemy vehicles or large groups of infantry, but is only provided once throughout the entire campaign. The target locator must recharge after each shot. It has a variable zoom of 3x and 6x.