Having watched the episode "Semper Fidelis" again, I've found that the Marine NCO only has that HK416 in once scene in the episode, that being the stand-off scene where the majority of screen caps were taken from. All other shots I saw showed an M4 with a 14-inch barrel, fixed front sight, and no uninterrupted top rail.
Here's the cap from the article. Notice the D-ring between the receiver and RIS and the fixed front sight barely visible ahead of the PEQ-2 at the bottom of the pic.
I noted this on the talk page, but nobody seems to have seen it.