Four-forty

A 440mm (or "Four-Forty" as referred to by UNSC troops) is a UNSC weapon developed around 2552. As an artillery piece, it is most likely used to destroy Covenant forces from a distance, providing indirect fire support to Marines. During a training mission on Onyx, one of the SPARTAN-III candidates mistook an explosion caused by an Onyx Sentinel for a 440mm piece. It may be an explosive shell, or, more likely, an artillery rocket. It was recently introduced prior to the Battle of Onyx.

Size Issue
The size of the projectile causes something of a discrepancy. If it is indeed 440mm, then it is unnecessarily large for an artillery piece especially given the UNSC's use of missiles and rockets instead. If it is .440mm, then it seems too small. However, if the .440 artillery uses mass driver technology, like that of Harvest, then the size makes much more sense - the kinetic energy imparted by a railgun or coilgun means that the projectile does not need to be particularly large, or even explosive, to inflict tremendous damage upon a target. If it is 440mm, then it may be a ground-based anti-ship weapon. For now, either theory remains conjecture. When contacted, even Eric Nylund could not explain such an oddity. He mentioned, however, that a 440mm cannon could be used against orbital installations. This however, would only make sense if the weapon was a direct-fire anti-ship magnetic cannon. A .440 magnetic cannon COULD NOT be considered artillery, because it would not be an indirect fire weapon, leaving the 440mm as the only possibility.

It could entirely be possible that it is a Mortar weapon, similar to large scale World War 2 Mortars, like the 380mm Sturmtiger or the 600mm Karl-Gerät

Trivia

 * On the military base, Reach, they called them forties-forties.