Is it legal to use an M-16 bolt carrier (full auto) in an AR-15 (semi-auto) with a semi auto trigger group?
        I are smart, but not that smart

I is really smart too. I is smart enuff to know that a full auto bolt carrier will NOT make an AR fire full auto. However, the ATF is not as smart as me and you and says it is illegal.

lol, thank you smart Guy. What would one do with such a bad rifle component?

lol, thank you smart Guy. What would one do with such a bad rifle component? That is if one were to have such a component, hypothetically of course

Supposedly if it is an M-16 part it makes your semiauto a "machine-gun".  Of course it is no such thing, but if BATF gets a hold of it they will disable the disconnector and get the thing to double a couple of times and you'll be screwed cause you were in possession of an illegal machine gun.
Since a bolt carrier is a relatively expensive part, you might consider grinding off some of the tail part that is supposed to trip the auto-sear.  Then it isn't an M-16 part anymore.  

Weiseguy, that would be to extend the cut out on the bottom further back? How much would I need to grind? Thanks for the info.

Yeah, just grind off the front part of the bottom part of the tail about 1/2" or so and you will be fine.

Also, if I hypothetically had an M16 bolt carrier in my rifle, it might be hypothetically a good idea to make sure I had no other M16 parts in it.

Thank you muchly, you guys are most helpful. This hypothetical carrier would be in like new condition, so hypothetically it would be worth altering it I think, in that hypothetical situation.  :o)

Myth.  Only if the part is designed to be used "solely and exclusively" to convert semi into FA will it be considered a MG.  The federal code section basically says just that in a nut shell.  [i]see[/i] 26 USCS 5845(b).  So, one MG part in your AR illegal?  Not if you use it as a working part of your semi AR and not to convert.  However, I would not do it 'cause it's just not worth it.  

BTW, some LE, self-proclaimed experts and Bushmaster will state unequivically that one part makes a MG.  Like I said, IMHO, the code section simply does not say this.  My advice is the same, however, don't chance it- it's not worth it.

We got into a long drawn out discussion on the email list (back when that was working) over if one part made it illegal or not.

As you said the law actually states [i]any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in [b]converting[/b] a weapon into a machinegun[/i] (emphasis added on converted by me)

By going by that alone with no other influence, it would seem to be legal to have a 100% M16 just without an autosear. M16 hammer, trigger, dico, selector, and carrier. Without an autosear it still won't fire auto. It might be possible to make it double by selecting it to auto, thereby diengaging the dico, however I have had a disco fail to engage between 5 and 10 times before and it never once doubled. I was pretty surprised when the hammer was down on a live round. Simple fact is, at least with mine, that there just isn't enough energy left when the hammer rides the carrier.

Anyway, as Steve said it kind of becomes an iffy issue since the ATF can turn nothing into something. It's highly [u]not[/u] recommended though. Grinding 1/2 inch of carrier back and you will show your intent to not have a FA gun should the ATF ever end up knocking at your door. And for that matter a bunch of uninformed people or cops won't be screaming to high hell if you open it up at the range and they see an M16 carrier.