You clearly do not understand what makes the Lockhart Raven 5.56 sufficiently different from an AR15 that it received a NR FRT entry instead of being classified as a Prohibited Variant. The ATRS Modern Sporter used a proprietary Receiver set but full AR15 internals, which ultimately resulted in a Prohibited Variant classification. The Raven 5.56 uses a proprietary Receiver Set as well as a proprietary Bolt-Carrier, Firing Pin and Charging Handle. This is because Samuel Lockhart decided to lengthen the Receiver Set by approximately 15mm, requiring those specific internal parts to be lengthened as well. Seeing as how the only thing that sets the Raven 5.56mm apart from the ATRS Modern Sporter is the lengthened internals, I'd have to insist that the proprietary Bolt-Carrier is the key difference that resulted in a NR classification for the Raven 5.56.....
Oh, and once you understand that the Raven 5.56 Receiver and Internals were lengthened to receive a NR classification, you will realize that would be pointless to add a standard BCG to the Lockhart rifle. The Carrier and Firing Pin would be 15mm too short to function in the Raven. I suspect that the whole reason the Bolt Carrier is skeletonized in the first place, is to offset the additional weight of the lengthened Carrier Body. But then what do I know with my limited knowledge of the AR15/M16??? LOL....