There has been a HUGE thread on this in the Legal section. I have commented, and got flamed for it by people who don't comprehend, that it may very well be perfectly legal under the letter of the law, but I would just as soon not have any M16 parts because BATF could easily start trouble. The lawyers very well may be right about the specifics of the law. But, if BATF comes to YOU, with some jackass with a badge saying the law is what he says it is. Oh, sure maybe you'll find your way through the system and be exonorated - not 100% odds, as they don't like to be wrong. But, you will have spent a fortune and a lot of time. Like I said, I got flamed for it, but anyone who has had anything more than a traffic ticket knows that you get as much justice as you can afford to buy.
So, yes it is legal. But...
Some say a DIAS is legal, too, but there was another thread in which it came up and one of the knowledgeable people on there cited title and section where it was NOT.
You either play the game, or keep it clean. Or, keep it clean, and be ready with what you need for another time and situation.