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 bases, or possibly to target vehicles. During a training mission on Onyx, one of the SPARTAN-III candidates mistook an explosion caused by an Onyx Sentinel for a .440. It may be an explosive shell, or, more likely, an artillery rocket. It was recently introduced prior to the Battle of Onyx.

When contacted, even Eric Nylund could not explain such an oddity. He mentioned, however, that a 440mm cannon could be used against orbital installations.

Size Issue
The size of the projectile causes something of a discrepancy. If it is indeed 440mm, then it is unneccessarily 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.