CZ 805 BF4 Mod bf2 . The CZ-805 BREN is a Czech assault rifle with a modular, multi-caliber design, with an aluminum alloy upper receiver and polymer lower receiver / fire control unit. The magazine housing is a separate detachable unit, which can be replaced in the field as part of a caliber change. The weapon is gas-operated and features a rotating bolt and a manual gas regulator.