I know this has been discussed in the past, but a comment in a recent post caused me to ask again.  (I've done a search of posts and read the materialin the Information Section already.)

In a discussion about Bushmaster vs. Colt "patrol" carbines, one of the pro-Colt responses stated that Colt was using the "M16" carrier.  I recently acquired a couple of nice Colt M16 bolt/carrier sets and intended to use one in a build.  I understand the physical differences in the M16 vs. AR15 carriers and was planning to modify mine.  However, even after reading the material in the Information files and the ATF FAQ, I'm not sure what is required.  I also remember some discussion of this recently.  Can someone set me straight on this discussion.  I'm specifically interested in the carrier issue.

Thanks for any insight and for the patience of the posters if I have inadvertantly released some demons.  8 )

Most say that it is not necessary to modify them at all for the sake of legality. But if you want to, all you need to do is grind away some of the part of the carrier that trips the auto sear in an M16. It will be the front edge of the back section when the carrier is turned upside down.

Thanks.  I have a handle on what to do with it, probably mill it back to the dimensions of my RRA "enhanced" carrier.  But I'd rather not be grinding on a bolt if not necessary.  The discussions and illustrations in the Info section are pretty blunt, but not consistent with my read of the BATF FAQ.  I was particularly curious when someone mentioned "M16" bolts in current Colts.

I will welcome any further suggestions.

I believe there is a multi-page tacked thread in the Legal forum that addresses this issue.

ETA: www.ar15.com/forums/topic.html?b=1&f=6&t=147440

Quoted: But I'd rather not be grinding on a bolt if not necessary.

There is no such thing as "an M16 bolt."  The carrier can be different, but the bolt is the same.  DO NOT do anything to the bolt itself.

The legal issue really boils down to whether the cutout in the bottom of the bolt carrier is short enough to trip an auto sear, and whether or not this is allowable.  From everything I've seen, that one part is not a legal issue.  BATFE has a history of being vague and less than forthcoming with details, so it's quite reasonable for you to have some questions about what's legal and what isn't.  Check the thread posted in BSBG's post above for a DETAILED discussion of the subject.

Thank you very much.  I was aware that this topic has been beaten severely, but I couldn't find a discussion thread.  I'm now more educated.  

  The carriers that are coming in the new ARs are M-16 carriers, without the required goodies in the rifle or in your possession in any way you are good to go. In and of themselves they do nothing but add a bit of weight. Have fun and don't worry.