A GBBR would require a buffer tube, Wilkie.
A buffer tube on an AR is where the bolt moves into during the blow back (or when you rack the gun), to chamber a round and cock the hammer.
That ICS gun is a "concept" gun, and would not work with a real M4.
I can show you on my GBBR at the TTAC game.
That G&P is beautiful, though.
