Talked with my gunsmith the other day. He's retired navy, and a former police armeror. He said it was the potential goo buildup (mentioned by teddy12b below) that could cause problems in the gas tube (also bolt lugs/recesses) if not cleaned thoroughly. He suggested using long pipe cleaners to clear the gas tube. Also, on the AR15 forum, a couple of guys suggested using a shot of brake cleaner in the gas tube. I think I'll use the Outers Foaming Bore Cleaner, followed by a shot of brake cleaner and then the long pipe cleaner.
I knew that the Outers' stuff was non corrosive, non abrasive, no ammonia and no harsh chemicals. I have used it with complete satifaction on other rifles.
Thanks for all the replies.
Paul