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
This prototype Battle rifle was also tested at a reach military facility codenamed "king under the mountain".
 * 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.