It kind of looks like the plastic piece that retains the large bolt for the buffer plate in a G3 stock assembly. It goes inside the plastic stock, and can only be removed if you also remove the pin holders because that triangular tab is blocked by the pin/butt pad retention tubes.Some kind of buffer part?
Simple - tiny bullet, huuuuuge primer. People just aren't forward thinking enough to recognize the innovation so they call it an "error" or a "stunt"...Not in, but I still want to play. I've always been remarkably impressed with HK's innovation. They sure seem to be steps ahead.
How do they do it? I don't even know how this works but it seems neato!