A Magnetic Accelerator Cannon (or MAC Gun) is a very large coilgun in the Halo universe. The cannons fire metal slugs using a system of magnetic fields down a long shaft, increasing the projectiles' 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 launch 3000 metric ton rounds at four-tenths (40%) the speed of light—that is, 120,000 km/s—giving a kinetic energy of 2.2 x 1022 joules or 1.6 x 1022 foot-pounds force. The Mk. V's could also reload in five seconds.
MACs can also be mounted on ships, such as the destroyer UNSC Iroquois, which had two cannons on board. The cruiser Pillar of Autumn was outfitted with new MAC technology: Magnetic field recyclers coupled with booster capacitors allowed the cannon to be fired three times per full charge. The Pillar of Autumn also carried new lighter MAC rounds that had the traditional ferrus core but a harder outer layer of tungsten carbide. Shipborne MAC Cannons draw power from the ship's reactor and require a charge to be fired. The time it takes to bring the MAC's systems to full charge on a UNSC vessel is the deciding force in a conflict. Thus, while a vessel may not run out of ammunition for a long time (depending on the size/weight of the shots and the capacity of the ship), a captain or commander has to carefully analyze the situation of a battle and use his shots strategically.
All Magnetic Accelerator Cannons are built into the structure of a ship or weapons platform. Thus, to aim the cannon, the ship or the platform must maneouver to be facing the target. Both shipboard and platform versions generally require an AI to aim the cannon, as the slugs fired are unguided. Even taking into account the high speed of the shots and the supreme accuracy with which an AI can aim them, they are highly innaccurate as battles usually take place across an entire system, allowing Covenant vessels to dodge them.
Twenty orbital Super-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:
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 Cairo Station, Malta Station, and Athens Station . 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! We've 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 UNSC In Amber Clad, Commander Miranda Keyes' ship.
Modified MAC Cannon
When the Pillar of Autumn is reserviced for the struggling war against the Covenant it receives the latest version of ship based MAC weaponry. This new version of the MAC Cannon has the ability to fire three successive rounds when coupled with power boosters from the ship. The commander of the ship Captain Jacob Keyes tells us that this will bring great strategic use to him as the first or second shot will down the Covenant ship's shields and the third shot (providing the shot will penetrate the heavily armoured control room located at the center of most Covenant spacecraft) will deliver the 'knockout'. This technology is only possesed by the Pillar of Autumn and is never seen in gameplay.