So is it safe, or a super-de-dooper no-no to share bolt assemblies between uppers?

Ill jus pull this outa my ass, but from what i have been told , if ya use a brand new bolt, ya can put it in any upper. But if ya use a used bolt in an upper, other than the one the original bolt came from , ya could have LOCK UP issues, due to the bolt seating properly. I personally haven never exchanged bolts, but am lookin to have a full bolt assembly as a spare.

You need to buy some headspace guages before switching used bolts around. As above, I *think* its pretty safe to slap in a brand new bolt in a brand new upper.

Bolt carriers dont matter. Swap the carriers all you want.

The rule is if the bolt or barrel has less than 500 rounds on it, it's still considered "new" for this particular purpose.  When I'm talking about using two used parts together, I'm NOT talking about two parts that were together since they were new.  I'm talking about using a used bolt from a different rifle with a used barrel from another rifle.

-It is ok to put a new bolt with a new barrel.
-It is ok to put a used bolt with a new barrel.
-It is ok to put a new bolt with a used barrel.
-It is not ok to put a used bolt with a used barrel.

That is by the book.. now for the gray area:

With #2 & #3 you still have to use common sense.  If the lugs are terribly worn on the used part (bolt or barrel extention), I'd be hesitant to use it.  Basically, if the used part is borderlining needing to be replaced but is still "good" for now, I'd recommend you just replace it now.

Now, as for the last one..  supposedly, you MIGHT have a failure to lock or other issues.  I still think that if there is not significant visible wear to the lugs of the used bolt or used bolt extention you'd be safe using the two together.

I swapped in a used bolt, no problems.

The last time I shot my M16 on Full Auto I had the company of a Bushy shooter.  When a barrel would get excessively hot we would change out the upper receiver.  We would also swap the M16 bolt assembly.  So while the barrel would change, the lower and the bolt assembly would always be together.
And having said all that, there were no problems of any kind that day.  My Colt lower borrowed a Bushy upper with no problems at all.  A Colt bolt carrier became well used in a Bushy upper too.

So unless you are talking about super accurate shooting, I really don’t know what the big problem is.

Stupid question here, but haven't seen it and was asked of me (I keep all assemblies together to avoid headache)

If you do put new bolt in "well used" bbl, I know there is acclerated wear from the 'rounding' of the edges in the lugs.

but, how do you adjust the headspace properly?  shims on barrel during install?

For the last time there is no adjustment for headspace.

Naw, David, it's not the last time---only the last time until the next time[:D]