Four-forty

A 440mm (or "Four-Forty" as referred to by UNSC troops) is a UNSC weapon developed around 2552. As an anti air 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 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.

The true caliber, out of the suggestions appearing above, is obvious to anyone with any real-world military history knowledge and experience, and a basic grounding in physics; the "440" no doubt refers to millimeters. While reading the previous conjecture, keep in mind that the .440mm projectile suggested, being a SUB-MILLIMETER projectile moving at a significant portion of the speed of light (compared to contemporary weapons), would be traveling LINEARLY if fired from a mass accelerator, not BALLISTICALLY. Field pieces - ALL field pieces - are, doctrinally and functionally, INDIRECT-fire weapons. As canon examples for your comparison, the MA5 series of rifles, the Gauss cannon, and most other projectile weapons in the series are DIRECT-fire weapons. Historically, weapons as large as a 440mm piece (17.5 inches, a little larger than the biggest guns conventionally fitted to battleships) were used as preparatory fire prior to ground invasions, in the destruction of hardened military installations, and in conjunction with and supplementary to air support and missile attacks. Weapons over twice this size were created during World War II as siege and assault weapons, and were capable of delivering multi-ton high-ex projectiles over distances of greater than 20 miles. These weapons also lack the weakness inherent to electronically targeted and guided systems; due to their mechanical nature, they are rather obviously unaffected by any form of electronic countermeasures and are exceedingly difficult to neutralize even with "point-defense" CIWS systems, due to the ballistic trajectory and the speed of the terminal approach. These concepts jibe well with the expeditionary nature of the UNSC Marine Corps, and with the engagement methods presented to us in series canon. With weapons such as these deployed planetside, it would be possible for a comparatively small, self-contained unit to provide for itself offensive and defensive fire to a substantial portion of ground forces over a wide area, much like their historical real-world usage. As an addendum, the author's suggestion to use a field piece as small as 440mm against an ORBITAL installation is absurd. A 440mm Linear Cannon (read: a futuristic MAC Gun that would have the necessary escape velocity to reach an orbital installation and then cause kinetic damage) would make sense in context, but once again, we're talking about future evolutions of modern-day indirect-fire artillery pieces, NOT linear cannons.