I've been asked by someone to help him identified this weapon (and what is that thing on the end of the barrel), but I can't seem to find what it is
Sounds crazy I'm sure but it looks like a bull barrel AR-15.
That thing on the end looks like a gasblock.
But as to why it's out there I don't know.
That is indeed an extra gas block out there at the end.
I use the exact same gas block on my bull barrel builds.
They come from Brownells:
My best guess about WHY the gas block would actually be out there would be as a mounting platform for a front sight, much like is done on DCM match rifles.
As there is no rear sight, maybe he unbolts the front sight when he takes off the rear sight and puts the scope on there...