What's the difference between an AR15 and M16 bolt carrier group (if any)?


I read on GT (call BS if needed) that you should NOT use the bolt group from one rifle to another?  If that were true, why do I see used ones selling all over the EE?

Check this out.   http://www.ar15.com/forums/topic.html?b=1&f=6&t=147440

The M16 bolt carrier is slightly heavier and shorter in the bottom open slot to trip the auto sear.
Switching bolts, not bolt carriers, is not really a good idea. The head space from one bolt to another barrel may be out of spec. Best to keep a bolt with a barrel.
You can switch bolts between barrels and they could be within specifications and work well.
But it's just best to keep the same bolt with the same barrel. It was matched to that barrel extension for best speciifications (hopefully) so keep it that way if possible.

Quoted: Check this out.   http://www.ar15.com/forums/topic.html?b=1&f=6&t=147440

What a pissing match that turned into.

Quoted: The M16 bolt carrier is slightly heavier and shorter in the bottom open slot to trip the auto sear. Switching bolts, not bolt carriers, is not really a good idea. The head space from one bolt to another barrel may be out of spec. Best to keep a bolt with a barrel. You can switch bolts between barrels and they could be within specifications and work well. But it's just best to keep the same bolt with the same barrel. It was matched to that barrel extension for best speciifications (hopefully) so keep it that way if possible.

Assuming it is out of spec, what happens?  Will it simply FTF? Or could it damage something?

Too tight, the bolt won't close and you can't fire the rifle. Too loose, you can have a primer back out, jam, etc. - a multitude of things that will cause a FTF.
But most all are within specs - a tight chamber or loose chamber - and stil fire. Where you have problems is with used parts and that may not happen too often, all depending on the wear on the components. With new parts, a bolt at maximum size with a chamber/barrel extension at minimum size, and you might have too tight a chamber where the bolt won't close.
I wouldn't worry about it if you are using new parts from reputable manufacturers. It would be a really rare occasion that you will have problems with new quality parts.

Hmm....my ar came w/ the FA bolt carrier...

it wasnt until I looked at this thread did I know they were different.

That's one of the things that was in dispute and still may be as far as having ANY select fire parts in an AR. I don't keep up on the ATF technical crap anymore but lots of people have the select fire bolt carrier in their ARs. If you only have two pins and a 3 position safety/selector you can be busted for having an illegal AR, even if it doesn't have the other parts required for select fire.
It's all bull hockey as far as I'm concerned but you have to follow the ATF rulings. I've been told the FA bolt carrier is OK JUST by itself.
The heavier bolt carrier of the FA bolt carrier will slow down the cycle time, slightly change the recoil, and so on. Some guys I know that have built match ARs use them.