The Magnetic Accelerator Cannon (or MAC) is a fictional large coilgun in the Halo universe. These weapon systems are mounted on most UNSC capital ships - destroyers, cruisers, frigates, and carriers. Larger versions (nicknamed 'Super' MACs or the big "stick") of these are used as orbital defense platforms. 
The theory behind the operation of a Magnetic Accelerator Cannon is the same as that of a coilgun. The cannon fires a massive metal projectile using a linear system of magnetic fields coils down a long shaft, increasing the projectile's velocity until it carries an incredible amount of kinetic energy. The ship-based models use ferric, ferrous, or depleted uranium cores, while orbital platforms and station-based versions use ferric tungsten rounds.
Magnetic Accelerator Cannons are built into the superstructure of a ship or weapons platform, requiring that it maneuver in order to aim the weapon. Shipboard versions generally require an AI to aim the cannon, as the projectiles are unguided. Orbital platforms have dedicated targeting computers.
They are the only non-nuclear weapons in the UNSC arsenal capable of effectively reducing or destroying Covenant shields. Smaller shipborne versions can take as many as 3 hits to overload a shield while an orbital platform can put a hole through any size Covenant vessel up to, and including a carrier.
Standard Magnetic Accelerator CannonEdit
The standard ship-mounted MAC system fires 600-ton depleted uranium rounds. An average UNSC frigate sports one MAC system, while a standard destroyer may possess two. UNSC cruisers and carriers carry an unknown number of these powerful projectile weapons.
Shipborne MAC draw power from the ship's reactor and require a charge of the weapon's magnetic coils in order 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. The weapon can be fired even when not at full charge, but the range and velocity of the projectile is greatly diminished.
When the UNSC ship Pillar of Autumn was refitted for the war against the Covenant, it received the latest version of the ship based MAC weaponry. Magnetic field recyclers coupled with booster capacitors allowed the cannon to be fired three times per full charge, unlike traditional shipborne MAC systems, which could only fire once per charge. 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 will deliver the final blow. The Pillar of Autumn also carried new lighter MAC rounds that had a ferrous core but a harder outer layer of tungsten carbide. This technology is only possessed by the Pillar of Autumn and is never seen in gameplay. Also, the Pillar of Autumn's MAC charges up faster than normal UNSC MAC cannons, most likely due to the Pillar of Autumn's advanced reactor.
Super Magnetic Accelerator CannonEdit
There are at least two versions of the orbital MAC platforms. The twenty Super MACs at Reach were designated Mark Vs and could launch 3,000-ton ferric-tungsten rounds at four-tenths the speed of light (268,246,651mph) giving a kinetic energy of over 5 teratons of TNT. The Mk. V Super MAC systems could also reload in five seconds. They were directly powered by a fusion generator planetside, which meant that if the generators were sabotaged or destroyed, the Mk. V would be inoperable, as described in Halo: Fall of Reach. The Super MAC Platforms protecting Earth may be powered by a different method as no visible link to the ground is observed, but it is speculated that power transmission is used.
In the vaccuum portions of the level "Cairo Station" on Halo 2, it is possible to witness the station firing the its Super MAC Cannon.
Twenty orbital Super-MAC Guns 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 past three hundred. They are grouped into clusters of three. Some of the more famed Super MAC Orbital Stations above Earth are: Cairo Station, Malta Station, Athens Station and Nassau Station, each in geosynchronous orbit above the cities for which they are named. The first three belonged in the same cluster, but following the events of Halo 2, only the Cairo platform survives. A single hit from one of these large weapons is able to destroy a Covenant capital ship.
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 Jacob 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 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.
- ↑ Nylund, Eric (November 2001) . "12, 16, 29, 31, 33" (in English). Halo: The Fall of Reach (First ed.). New York: Random House. pp. 107, 139, 275, 283, 296. ISBN 0-345-45132-5.
- ↑ Nylund, Eric (November 2001) . "12" (in English). Halo: The Fall of Reach (First ed.). New York: Random House. ISBN 0-345-45132-5.