The HMG

The AIE-486H Heavy Machine Gun is a standard Gatling-style heavy machine gun, used by the UNSC for offensive and defensive purposes. It is also more powerful than the UNSC's stationary turret, the M247 GP Machine Gun. It has saved many lives with its defensive capabilities holding enemy forces at bay for a limited time during the battle of Reach, accounting for many casualties to the Covenants efforts to take out the Orbital MAC Generators.
For defensive purposes, it can be mounted on a tripod, but when used for an assault it can be irreversibly and quickly detached from its tripod by a SPARTAN-II or Elite's brute strength. In its unmounted form, the weapon has a limited ammunition capacity of 200 rounds and takes about a second to "spin up" before reaching its highest rate of fire. Pulling the trigger once will cause the gun to fire a maximum of two rounds at a moderate rate; holding it down builds up the rate of fire to a higher rate.
The gun slowly climbs during automatic fire like the M7 Caseless Sub Machine Gun, but its recoil is lessened by the gun's weight, which makes it hard to move around as well as decreasing the user's speed. Unlike most mini-guns in other games, the AIE-486H does not overheat with continuous fire. This is a more realistic feature as having three barrels on the gun disperses the heat equally within each barrel; allowing the gun to remain cool and continue functioning.Ammunition
The AIE-486 fires 7.62x51mm Saboted Light Armor Penetrator rounds at a high rate of fire. The SLAP design incorporates a polymer sabot, which allows for the use of a tungsten penetrator projectile of a lesser diameter than the original bore. By using the casing of a large cartridge with a lightweight projectile, the velocity of the projectile is greatly increased. The bullets are powerful enough to punch through the armor of a Warthog or a Wraith and destroy it with ease.I have got this info from halopedia.com