Magnetic Accelerator Cannon
From Halopedia, the Halo wiki
Summary
The Magnetic Accelerator Cannon or MAC gun is the standard main weapon of all UNSC cruisers. It uses huge magnets that condense a molten sphere of super heated ferric-tungsten projectiles, and fires it at near-light speed. The damage it inflicts it tremendous, though reloading takes quite awhile. A more effcient, experimental model of the MAC gun, which can fire three rounds before reloading, was mounted on the Pillar of Autumn. Super MAC guns are loaded on the defense platforms orbiting Reach, Earth, and other strategic locations that must be protected, and are much more powerful than normal MAC cannons.
The MAC cannons can be found throughout the Halo universe, and play critical roles in many battles. The use of the MAC Cannon proved decisive when the Covenant fleet descended to earth at the beggining of Halo 2, and was used as early on as Master Chief's first encounter with the Covenant, in which it was used to blow a hole into the ship through which Spartan's could access.
