Magnetic Accelerator Cannon

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"That MAC gun can put a round clean through a Covenant capital ship."

- Sergeant Johnson, talking to the Master Chief on board the Orbital Defense Platform Cairo.

The Magnetic Accelerator Cannon, also known as Mass Accelerator Cannon, MAC Gun and MAC Cannon, is a large coilgun that serves as the primary offensive weapon for UNSC warships. Larger versions, nicknamed "Super" MACs or the "big stick", of these are used as orbital defense platforms. 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 three hits to overload a shield while an orbital platform can put a hole through any size Covenant vessel even with fully charged shields.

Overview
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.

Mounting
The size of a Magnetic Accelerator Cannon is such that it is normally an integral component of a warship's structure. Some types of Defense Stations are literally built around the massive weapon for orbital defense.

Ammunition
A standard ship-based MAC fires slugs of either ferric tungsten or depleted uranium at around 40% the speed of light. The high muzzle speed gives the 600-ton slug the kinetic energy and momentum necessary to damage a target and partially mitigates the unguided nature of the slug and its lack of maneuverability. Orbital Defense Platforms fire a 3,000-ton slug at nearly 50% of the speed of light, around 150,000 km per second, which is capable of piercing multiple obstacles before fully stopping the projectile, if needed.

Aiming
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.

Firing
The firing process uses electromagnetism to fire a ferromagnetic-tungsten slug at high velocity. An extremely large amount of current is put through the first solenoid (coil of conducting wire) which creates a strong magnetic field which attracts the metal slug. As the slug passes into the solenoid, the solenoid is quickly turned off and the second solenoid, which is further up, turns on, which attracts the now high velocity metal slug just like the first solenoid, and the process is carried on. By the time the slug is fired out the end of the barrel it has been accelerated to a speed of approximately 120,000 kilometers per second for ship-based MAC and around 150,000 kilometers per second for "Super" MAC. At the same time, a pair of thrusters on the bottom side of the station fire for a couple of seconds to counteract the acceleration imparted to the station. It normally takes five seconds to recharge the capacitors as well as load the slug, which is why boarding craft have to be deployed to take the stations out.

Standard Magnetic Accelerator Cannon
The standard ship-mounted MAC fires a 600-ton ferric-tungsten projectile with a depleted uranium core at 120,000 kilometers per second. The large amount of energy needed to fire the weapon is particularly onerous on a warship, and the extended recharge time is a significant factor in combat against Covenant warships as multiple MAC rounds are required to penetrate Covenant shields. At 1.17 Teratons per shot, the standard Magnetic Accelerator Cannon is sufficient to destroy any human vessel or severely damage an unshielded Covenant vessel.

Shipborne MAC draw power from the ship's reactor and require a charge of the weapon's magnetic coils in order to be fired. According to Halo: the Fall of Reach it takes three shots form the standard MAC cannon to effectively lower the shields of a covenant ship and destroy it. 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.

An average UNSC frigate sports one MAC system, while a standard destroyer may possess two. UNSC cruisers carry two and carriers carry an unknown number of these powerful projectile weapons.

Modified MAC
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 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 Cannon


A UNSC Defense Platform typically mounts larger and more powerful version of the standard Magnetic Accelerator Cannon, nicknamed "Super" MACs or the "big stick".

These cannons fire a 3000-ton ferric-tungsten round at nearly 50% the speed of light, impacting with a massive amount of relativistic kinetic energy, which at 50% of the speed of light is of equal power to 9.98 teratons of TNT. No known ship, UNSC or Covenant, has been shown to survive the impact of one of these rounds. Against Covenant shield technology, the rounds possess enough kinetic energy to punch through shields, cut through the ship, and, upon exit, still retain enough energy to cripple or destroy a second ship.

It is theorized that if a ship's armor or shields were to absorb all the kinetic energy of a MAC round, the release of thermodynamic energy would still vaporize the ship. By receiving power from ground-based power plants, orbital platforms could achieve recharge and reload times as short as five seconds.

M68 Gauss Cannon
The M68 Gauss Cannon is a UNSC weapon mounted on the M12G1 Warthog LAAV and on the stationary Gauss Turrets. It utilises the same coilgun technology used in Magnetic Accelerator Cannon, but on a smaller scale.

M99 Stanchion Gauss Rifle
The M99 is a man portable anti-material and anti-personnel special application weapon. Like the M68 Gauss Cannon, it utilizes the same coilgun technology as a MAC, only on an even smaller scale, using 5.4mm/.21 caliber rounds. Though smaller than conventional ammunition, these rounds are accelerated to speeds fast enough to travel long distances with almost no change in trajectory, punch through any obstructing materials, hit their target, and still retain significant kinetic energy.

M92 Principle Gauss Cannon
The M92 Principle Gauss Cannon is a Magnetic Accelerator Weapon that runs along the length of a Open Frame 92/Extra-Vehicular Activity.

Mass Drivers
A Mass Driver was a somewhat primitive United Nations Space Command orbital launch assembly designed to propel low-weight payloads into orbit, otherwise known as orbital lifting. The operating principle of the Mass Driver is essentially the same as that of the Magnetic Accelerator Cannon: a coilgun that magnetically accelerates a package consisting of a magnetizable holder containing a payload.

While not a purpose-built weapon like the UNSC's Magnetic Accelerator Cannon, these platforms are still potent weapons of opportunity. While never officially incorporated as a weapon, Mass Drivers have been used, most notably at the Rubble and Harvest, as defensive magnetic accelerator weapons.

Other MAC Sizes
Although never explicitly elaborated upon, there may be many more different sizes of MAC weaponry based on the sizes of possible mounts. For example, one might extrapolate that a frigate would mount a smaller, less powerful cannon than a supercarrier or a, if only for power management issues. Conceivably, a 'large' ship could mount many 'small' MACs. However, the lack of such occurrences might suggest that the power of a MAC, once the mass and volume of all attendant components (i.e. capacitors) are accounted for, does not scale linearly. There is one known exception (destroyers), which are outfitted with two MAC cannons. The Pillar of Autumn had one large MAC cannon which could fire three 144 ton "shredder" rounds in succession on a single charge (see below).

MAC Location
Many believe that the MAC barrel is located near the underside of the ship, where there are three large plates. However, it is also quite likely that the barrel is located along the entire length of the ship.

Trivia

 * The Magnetic Accelerator Cannon is twice mistakenly referred to as the "Mass Accelerator Cannon" in the Halo Universe - in Halo: Contact Harvest, and in Halo Wars, in which it is referred to as a "Mass Accelerated Cannon", which seems to be a simple typo as it does not make sense grammatically. It may be a confusion between the MAC and the Mass Driver, a non-military application of similar technologies.
 * The colony ship-turned-warship UNSC Spirit of Fire's MAC cannon is revealed to be unable to penetrate a Citadel's shield in Halo Wars in one hit. This might be the result of a weaker, earlier version of the Cannon, or that the Shield is very powerful. It is likely though that the Spirit of Fire, being a colony ship refitted for battle, had a weaker MAC gun. Another possiblility is that since its MAC gun was designed for ground support and not ship-to-ship engagements, it may fire lighter MAC rounds, as normal sized rounds would be too powerful and would cause friendly causualties. This also could explain why the cannon could fire so many shots in quick succesion. Alternatively, it may only be for gameplay purposes.

Internal

 * M68 Gauss Cannon
 * MAC Blast

External

 * Halo.bungie.org: Overview of an Orbital Defense Platform by Stephen Loftus
 * Wikipedia's article on Coilgun

MAC