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 battle rifle most likely uses the 60 round magazine from the colonial milita's MA5s.