Magnetic Accelerator Cannon

The Magnetic Accelerator Charge (or MAC Gun) is a large coilgun in the Halo universe. These weapon systems are mounted on all U.N.S.C. capital ships - destroyers, cruisers, and frigates. Larger versions (nicknamed 'Super' MAC Guns) of these are used as orbital defense platforms.

The theory behind the operation of a Magnetic Accelerator Charge is the same of that of a coilgun. The cannon fires heavy metal projectiles 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, ferrous, or depleted uranium cores, while station-based versions use ferric tungsten rounds.

All Magnetic Accelerator Charge guns are built into the structure of a ship or weapons platform. Thus, to aim the cannon, the ship or the platform must manoeuver 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 so, the high velocity of the round ensures at least 80% accuracy. The MAC gun orbital platforms have powerful targeting computers that allow them to accurately target a Covenant ship. An EMP (electro magnetic pulse) would short them out, however. The Covenant learned this over time in numerous battles with the U.N.S.C. and engaged the fleet over Reach at point blank range, rendering the human's nuclear weapons useless (Due to the fact that they could not use their SHIVA nuclear weapons, or they would burn out the Super MAC gun coils.)

They are the only non-nuclear weapons that the UNSC possesses that can destroy Covenant shields quickly.

Standard Magnetic Accelerator Charge Gun
The standard ship-mounted MAC system fires 600-ton depleted uranium rounds. An average U.N.S.C. frigate sports one MAC cannon, while a standard destroyer may possess two MAC cannons. U.N.S.C. cruisers and carriers carry an unknown number of these powerful projectile weapons.

A pair of depleted uranium projectiles can destroy a Covenant frigate or destroyer.

Shipborne MAC Cannons 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.

Modified MAC
When the U.N.S.C. Pillar of Autumn was reserviced 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 gun charges up faster than normal UNSC MAC cannons.

Super Magnetic Accelerator Charge Gun
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 rounds at four-tenths the speed of light 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. The Super MAC Platforms protecting Earth may be powered by a different method as no visible link is observed, but some sources say that the power is transmitted.

You may also spot a Super MAC Gun firing in the Halo 2 game level 'Cairo'.

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 up to 300. 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 (The first three belonged in the same cluster, but only the Cairo platformed survived). A single hit from one of these large weapons is able to knock out 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 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.