(For UK campaigns)
Sterling "SMG" in 7.62×51mm NATO
If the worst ever happened Britain planned on making "Last Ditch" weapons such as the Second World War Primitiv-Waffen-Programm in Germany.
Thus this creature was designed and even one produced as a proof-of-concept. It is a full-auto/semi-auto lever-delayed open-bolt weapon that uses the Sterling SMG control group and L1A1 SLR or L4 Bren magazines. It could mount the L2A1 SUIT sight.
(Nobody knows its weight, ROF or accuracy and the MoD refuses to tell. It's probably still kept as plans and tooling).
