XBR55 battle rifle

Overview
The prototype BR-55 Battle Rifle (likely referred to with an X for experimental XBR-55, BXR-55 BRX-55, etc. etc.) is “a prototype long barreled weapon with an optical scope.” The weapon fires 9.5MMx40 FMJ SAP ammunition from a 60 round magazine It was used during the Harvest event by Staff Sergeants' Avery J. Johnson and Nolan Byrne as well as their militia squads' marksmen during the year 2525.

Trivia

 * The prototype Battle rifle has a 60 round clip.
 * This was the first weapon to be used against the Covenant and caused the first death to aliens by the Humans.