Four-forty

440 mm artillery piece or "four-forty" as they were refered to be UNSC troops, was recently introduced prior to the Battle of Onyx.

A 440mm 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.

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

The size discrepancy, being an apparently .440 round, poses a problem in terms of firepower. 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. This is one of several theories, but remains conjecture for now.