From Halopedia, the Halo wiki
The colloquially named "four-forty" is an artillery piece weapon used by the UNSC Armed Forces. It had been introduced recently as of October 2552. Spartan-III trainee Dante-G188 mistook a powerful, distant explosion on Onyx for the testing of a four-forty, which he claimed is a "big one"; however, whether he was referring to size, explosive yield, or both is unclear.
Though Halo: Ghosts of Onyx references the weapon as a type of artillery, its name is never explained. It is possible that the name is shorthand for a 4.40" (~112mm) bore, which would be appropriate for a fairly small artillery piece; however, this would conflict with Dante's description of the weapon as a "big one" unless the explosive yield is greater than the round's size would suggest. Alternately, "four-forty" may simply be a model designation such as "M440". A less likely possibility is that the four-forty is a large-scale mortar weapon, similar to Nazi Germany's 380mm Sturmtiger and 600mm Karl-Gerät.
List of appearances