So I'm flipping through the M16A2 operators manual and I come across a warning that  says...

*DO NOT exchange or switch bolt assemblies from one M16A2 to another.  Doing so may result in injury to, or death of, personnel.  

For someone working on a build this gave me pause.  What would be the danger?  Any mil-spec bcg should be good to  go, right?

You can change them but you have to check the head space.  Someone that knows more than me will likely expand.

I believe they are referring to one used bolt to another. The bolt mates to the barrel extension after so many rounds. If you swap them you may end up with a headspace issue which can be a dangerous situation.

Thanks.  I'm a newbie so I'm going to have to read more about headspace.

They really mean USED bolts.

The general consensus is that putting a NEW bolt in a used rifle is OK.  Putting a used bolt in is where the trouble lies.

The vast majority of the time, you will have absolutely zero problems switching BCGs, even used ones, between rifles.  

The people who write warnings in manuals are not concerned about the vast majority of the time - they are concerned about the .001% chance of catastrophic damage.  If the BCG started parked and now looks chromed, it's probably not a good idea.  

As well they should be, it's a litigious society we live in.  

I DO NOT RECOMMEND SWITCHING USED BOLTS INTO USED BARREL EXTENSIONS (see above).  

Swap at your own risk.  An excessive headspace situation could cause a KB.  I've still got all my fingers.  

~Augee

New bolt in used BE is OK, Used bolt in new BE is OK.  Used bolt in different used BE can cause excessive head space.  It can also cause broken bolt lugs.  The BE lugs and bolt lugs wear in and establish a contact pattern, changing one of these changes the contact area.  There's a remote chance of excessive wear or loading the may lead to failure. 99.9% of the time it's probably safe to do, but bolts are cheap and face surgery hurts.

It's to avoid infecting your new rifle with something your old rifle was carrying, most likely BRD. They don't call them bolt carrier groups for no reason. They're dirty, they've been around.

Depends on how used the bolt is dont forget bolts break in on the barrels extension so if a bolts really,really used you could have problems.