From Halopedia, the Halo wiki
The Sentry autocannon is a line of naval coilguns used by the UNSC Navy during and after the Human-Covenant War.
The M66 Sentry was mounted on the Epoch-class heavy carrier and Autumn-class heavy cruiser. The UNSC Pillar of Autumn was also retrofitted with them in preparation for Operation: RED FLAG.
A close up of a M66 Sentry on the Pillar of Autumn.
Mass drivers: Onager (Mark 2488 • Mark 2551) • Ballista