MAC Blast

Were you looking for the MAC Cannon?

"Use the Spirit of Fire's awesome Mass Accelerator Cannon to incinerate enemy troops."

- In-game description.

The Magnetic Accelerator Cannon Blast or Mass Accelerated Cannon, is a powerful type of ammunition UNSC ships use to bombard a ground area from orbit. In order to use this devastating weapon, the ship must point its MAC gun to the location. These strikes are capable of devastating relatively large areas, and can destroy even Covenant Scarab walkers with their strength, but you will need more than one shot.

Halo Wars
The "MAC Blast" is used by the Spirit of Fire to bombard an area of the ground from orbit, using its ship-mounted Magnetic Accelerator Cannon. It's mainly used as a support weapon to help the UNSC penetrate heavy Covenant defenses, big walker units, and Covenant bases.

Trivia

 * Although powerful, it is theorized that a real-life strike of similar proportions would rival nuclear warheads in their devastation. Instead, the MAC blast causes far less damage, though more than enough for the UNSC's needs, but they can take down Covenant energy shielding in space combat. They consist of a very heavy tungsten carbide round and the size varies, depending on the type of armament. The may use specially designed MAC shells, either smaller or lighter, for orbital bombardment, to prevent collateral damage to allied forces, or more likely, the rounds are being fired at a lower charge in order to fire so many rounds so rapidly.
 * The MAC Blast is similar to the Meteor God Power from Age of Mythology, an RTS game which was also made by Ensemble Studios and also published by Microsoft Games.