Yes, it's legal. The criteria is that a semiauto rifle that accepts a detachable mag can only have 1 feature from the 'bad' list if it was manufactured after 09/1994. The 'bad' list is things like flash suppressor, bayonet lug, pistol grip extending below the receiver, folding stock, and maybe others I can't recall. New AR-15's have the pistol grip, so they cannot have a flash suppressor. The M1A does not have the pistol grip, so it can have the flash suppressor as its one 'bad' feature.
