A MAC Cannon is a Magnetic Accelerator Cannon, from the storyline of Halo. The cannons fire metal slugs using a system of magnetic fields down a long shaft, increasing the projectile's velocity until they carry an incredible amount of energy. The ship-based models use ferric or ferrous cores, while station-based versions use either depleted uranium or ferric tungsten rounds. They are the only non-nuclear weapons that the UNSC possesses that can destroy Covenant shields.

There are at least two versions of the orbital MAC platforms. The 20 Super MACs at Reach were designated Mark Vs and could lauch 3000 ton rounds at "point four tenths" (40%) the speed of light - that is, 120000 km/s. The Mk. Vs could also reload in five seconds. The Orbital MACs in Halo 2 took more than five seconds to reload and their MAC rounds were nowhere near large enough to be 3000 tons (no matter how dense). Thus, the 300 stations orbiting Earth are likely Mk. IVs or below (since Earth pioneered the earliest orbital MAC prototypes, not Reach. Either that or Earth is still using an old version and Reach's Mark Vs were created recently).

MAC Cannons can also be mounted on ships, such as the destroyer Iroquois and the cruiser Pillar of Autumn. Shipborne MAC Cannons draw power from the ship's reactor and require a charge to be fired.

Both versions generally require an A.I. to aim the cannon, as the slugs fired are unguided.

Twenty MAC Cannons were used to defend the planet Reach from Covenant invasion, but they soon fell as the Covenant destroyed the underground generators that powered them. The planet Earth had 16, but production was stepped up quickly after Halo: The Fall of Reach, bringing the total up to 300. Some of the more famed Super MAC Orbital Stations above Earth are: Cairo Station, Malta Station, and Athens Station, each geosynchronized above the city they are named for.

In Halo 2, Cairo Station is the station where the Master Chief and Sergeant Johnson receive awards, along with Miranda Keyes, daughter of the famed Captain Keyes. When the Covenant come out of Slipspace, they attack each of these 3 stations. At one time in the middle of the battle, Malta Station seems to have won the fight, as they report: "I don't believe it. They're retreating! It's over, we won!" However, the Covenant soon demonstrates their new technique for dealing with the orbital MAC Cannons; using boarding parties to place a large bomb within the station. The Athens Station explodes shortly thereafter, as does the Malta and those aboard the Cairo come to horrifying conclusion that there must also be a bomb on Cairo Station. The Master Chief runs to find it and defuses it with Cortana's help. The Master Chief manually places the bomb in a nearby Covenant Capital ship and detonates it. The explosion propels the Master Chief through space to land on the In Amber Clad, Commander Miranda Keyes' ship.