440 mm Heavy Artillery

A 440mm, or "Four-Forty" as referred to by UNSC troops, is a UNSC artillery weapon developed around 2552. As an artillery piece, it is used to destroy enemy forces from a distance, providing indirect fire support for ground troops. It may be an explosive shell, or, more likely, an artillery rocket.

Background
During a training mission on Onyx, one of the SPARTAN-III candidates mistook an explosion caused by an Onyx Sentinel for a 440mm piece.