Below I have described in detail all of the UNSC, covenant, and Promethean weapons, including when and how to use these weapons. For the covenant and Promethean weapons, I have also provided the best strategies to overcome enemies with these weapons. The x/y after each gun name refers to the ammo per magazine/maximum extra ammo. Notice that the strategies are intended for use against AI in Spartan Ops and Campaign. These strategies do not necessarily apply when playing against human players in War Games.
“Headshot weapons” are a group of weapons with unique properties. A single headshot to an unshielded enemy from a headshot weapon deals infinite damage, allowing you to easily kill any rank of enemy on any difficultly. Headshot weapons are the most accurate weapons in Halo 4 and they all have scopes. This allows you to pick off enemies from a distance, which is usually the safest tactic on higher difficulties. You can also use headshot weapons up close, including against shielded enemies. Remove the enemies shields (preferably with an overcharged plasma pistol), and you then can finish them with a single headshot from a headshot weapon.
Assault rifle – 32/128: The assault rifle is the most accurate 5.7×28 ammo fully automatic weapon in Halo 4. Not only is it accurate, it also has very low recoil, so you can fire an entire magazine at once and still retain accuracy. When you do not have stronger weapons (such as the incinerator cannon or SAW), you can use the assault rifle to kill Promethean knights. Whenever possible, use either grenades or an auto sentry in combination with your assault rifle to more easily kill knights.
Battle rifle – 36/108: The battle rifle is an accurate headshot weapon with a 2x scope. It fires a 3-shot-burst each time you pull the trigger. These 3 bullets travel in a tight spread, increasing the area that you hit and allowing you to get a headshot even if your aim is a bit off. Because it fires 3 shots, the battle rifle is one of the highest damaging headshot weapons, so it is one of the best weapons for fighting Promethean watchers. Because watchers do not have heads, you must kill them with repeated body shots, so the higher damage of the battle rifle allows you to kill them in fewer shots. The downside to the battle rifle is that it consumes ammo very quickly.
DMR – 14/42: The DMR is an incredibly accurate semi-automatic headshot weapon with a 3x scope. It has practically no recoil and is just as accurate as any of the sniper rifles. Therefore, the DMR is great for picking off enemies from a distance. Using the DMR, you can easily kill light infantry from so far away that they cannot return fire. There DMR is also very effective up close if you do not use the scope.
Magnum – 8/32: The magnum is a secondary headshot weapon with a 2x scope. While it fires only one shot per trigger pull, the magnum has an incredibly high fire rate. However, I suggest that you instead fire slowly and carefully to conserve ammo and more easily get headshots. The magnum is not as accurate as the other headshot weapons, so you will need to get closer to your target when using it.