From Halopedia, the Halo wiki
The Castor naval coilgun is a large naval autocannon used by the Colonial Military Authority and United Nations Space Command. It is capable of being loaded with a variety of specialized ammunition, including experimental shield-piercing shells.
The Mark 40 Castor naval coilgun is employed on the UNSC and CMA Navy's Mako-class corvette. Two such cannons are employed by the corvette.[1]
The Mark 55 Castor naval coilgun is employed on the UNSC Navy's Strident-class heavy frigate. Five such cannons are equipped on the Strident, three of which are paired with two Mark 57 Arena point defense guns in a 'quickshot' configuration.[2][3]
The Strident's two starboard-side Mark 55 Castors.
Two Mark 55 Castor turrets on the Strident-class heavy frigate. The right-most is equipped in the "quickshot" configuration with two Mark 57 Arena PDGs.
Naval coilgun (LNT-450 • Castor) • Naval coilgun battery (11A2R1 • Mark 15 Breakwater • Spitfire) • Sentry • Gauss cannon (M65 • M68 • M68B • M70) • M92 Principle Gauss Cannon • Mini-MAC • "Super" MAC Mass drivers: Onager (Mark 2488 • Mark 2551) • Ballista