Well, don't know, to me it appears as a muzzle reference system, you know a thermal thingy that hooks on to the end of the barrel, when your barrel starts to droop from over heating , It allows you to compensate for the droop and remain on target. Could be wrong, I think I may have been wrong once or twice before.
Actually, I posted a picture of a gun that I thought would get some comment, but then found that I had posted on it before and didn't want to be reminded by some nitpicker that I was duplicating an old post. So I deleted it.