I have a VLTOR VIS-3A upper(rifle length). I'm in the process of a 6.5 Grendel build. I was going to get a Sabre Defence 20inch 6.5 Grendel barrel and bolt but the place I was going to get it from is out. They do have the 18inch barrel in stock. Will this work with the upper I have? My guess is that there will still be some barrel sticking out of the forearm, just 2 inches less.