Magnetic acceleration in combat.
Though Karl Friedrich Gauss is an unlikely subject of conversation in the corps, every UNSC soldier knows the basic principles of magnetic induction which the scientist pioneered. While complicated in detail, the central values behind such technology are fairly simple. Triggering magnetic energy currents in sequential order can exponentially accelerate an object, hurling it at an impressively violent speed. In scientific jargon, this process is called "asynchronous linear induction."
While such technology was used for transportation throughout the 21st century and beyond, only much later were machines such as mass drivers created. Using the same principle, mass drivers could launch payloads from terrestrial locations, ultimately delivering them into space at hyper-sonic speed.
It wasn't long after that the military began to explore a more strategic use of this technology, the earliest of such field applications were precursors to the Gauss cannon. And although using a magnetic field to channel high-density objects became an effective measure in terrestrial combat, it was even more potent in space, removed from atmospheric conditions and local gravity.
The most common planetside applications of magnetic induction are the UNSC's M12G1 Warthog LAAV and the SP42 Cobra, using M68 and M66 asynchronous linear-induction motors respectively. Magnetic Accelerator Cannons, or MACs, also use this technology. Housed on orbital platforms that surround specific worlds, these massive weapons are organized into battle clusters and provide a planet's first line of defense against outside attack.
Other less common, but notable applications include the M99 Stanchion, a sniper rifle that was used during Operation: TREBUCHET, and ONI's prototypic OF92/EVA, or Booster Frame, which was rumored to have been deployed in a handful of highly classified space operations.