I don't agree with the "SD" designation. "SD" signifies a dedicated "silenced device". Though this system has a long barrel shroud, it isn't "silenced". I would say that this weapon system appears to be a E-11 proto-type and should be designated as an "XM" (Experimental Model) followed by a #. That said, "XM" weapon systems aren't "normally" fielded to the rank and file but have been known to be utilized by Spec-Ops in field operations for feed back and re-evaluation. I just wonder how the writers designated this system?
Just my $.02 (two cents)...