My understanding is that the SCR can accept any AR-15 Upper. It obviously wouldn’t function with a standard AR-15 BCG as it doesn’t have a buffer tube/spring, but it seems like the current interpretation is that if an AR-15 upper can mate with a non-AR-15 lower, that makes it a variant and restricted. In a hypothetical situation where one had a NR SCR, it would be very easy to make it a prohibited firearm by popping on a prohibited length AR-15 Upper (and swapping out the SCR BCG), in likely less than 30-60 seconds and with no tools or machining required. Could very well be wrong about that though.