I've heard of people getting alternate uppers for their ARs and simply dropping them in to shoot using the carriers, bolts, and charging handles off their original upper.  I have been told the AR is "modular and as long as they aren't worn it shouldn't be a problem."  

I was told this is a good way to get an addl upper a bit cheaper because "you already have the carrier, bolt and charging handle on your AR."

"Modular" rifle system or not this makes me worry  that I will find out about excess headspace with a Kaboom...

Shouldn't I be getting a set of headspace gauges to make sure I am not getting too much or too little headspace on the new upper?  

(in which case I might as well get the upper with a fitted carrier from the vendor.)

AND

Why do vendors sell uppers without the bolt carrier if it may be a problem?

What am I missing here?

I would have a seperate bolt for each upper using the same carrier dosent matter but not the same bolt.

So I can use my carrier for all uppers I have but just buy a separate new bolt with the upper and keep it with that upper.

Does that mean headspace is ok as long as the new bolt will fully lock up?

OR

Should I get a set of headspace gauges to make sure when I buy the new upper? (I'd been told it is a good idea to have them anyway...)

Chances are 99%+ that any bolt you have, as long as it or the barrel isn't seriously worn out, will headspace just fine.  It's a nice assurance to headspace the bolt though - I do but that's because I have lot's of bolts and I got curious.  The headspace is set by the factory when they make the barrel and attach the barrel extension.  The military Colt field headspace gauge will only detect those that are really worn, and I mean WORN!!! as in tens of thousands of autofire.  I'm being kind of general.

Don't sweat it.  But if you have the gauge, by all means - check it out.  I have yet to come across a bolt that doesn't check out properly, but I don't buy crap.

Interchanging bolts always worried me too since the manuals make such a big deal about not swapping bolts between rifles, but a number of AR experts I have talked to say it is almost never a problem.

Assuming you've purchased a new barrel from a reputable maker, and are using a bolt assy from a top company, it should not be a problem.

FYI, a couple of years ago I purchased a bunch of 5 GI surplus bolt assys and tried every one of them in every one of my 4 (at the time) ARs.  These were all Viet Nam era M16 surplus.  All but one would not close on the NO GO guage, and the one that did close, wouldn't close on the FIELD guage.  That piece was so old, it still had a white extractor spring insert, which was replaced by the blue unit around 1967.

The bottom line? If you're using top quality stuff, don't sweat it[:D]