Four-forty

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 most likely 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.

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 coilgun or railgun technology, then the size makes much more sense - the kinetic energy imparted by an electromagnetic projectile weapon 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.

List of appearances

 * Halo: Ghosts of Onyx