Before free float hand guards were the norm I received a quality (not saying good quality) free floated 6x45mm upper from oLIMPIC arms. With no factory ammo available in 6x45mm I loaded a ladder from the book from bottom to top listed load. Nothing cycled, not even close.
So... back at home I disassembled it and found the barrel gas port/ gas block port were not lined up (the carbon on the barrel showed this). Repositioning the gas block forward to compensate for the "missing" front hand guard retainer that would have spaced the gas block properly solved the lack of gas.
So... My long way of saying, look at the carbon marking on the barrel for indication of bad port alignment.
So... back at home I disassembled it and found the barrel gas port/ gas block port were not lined up (the carbon on the barrel showed this). Repositioning the gas block forward to compensate for the "missing" front hand guard retainer that would have spaced the gas block properly solved the lack of gas.
So... My long way of saying, look at the carbon marking on the barrel for indication of bad port alignment.