XBR55 battle rifle

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 Bryne as well as their militia squads' marksmen during the year 2525.