I'm alittle confused about buying a upper without the bolt, carrier, charging handle. I was under the impression that you shouldn't swap bolts between rifles. However in the equipment exchange thats all you see, uppers without the bolt ect.. So whats the deal with this? If i buy a Bushmaster, Colt, or Rock River upper, will any new bolt ect.. work in it?, or should I avoid all the hassle and buy a complete upper. I have a stripped RRA lower already, and really look forward to my first build, just not sure about the upper. Thanks in advance.

If you put a new bolt in a new barrel you should be fine. It is always a good idea to check the headspace, though. I have seen an AR blow up (not too badly, no damage to operator) due to improper headspace.

just get a set of GO-noGO-FIELD gauges and make sure it locks on the GO and not the other gauges.
(it might lock on the field if you assemble some used parts). someone will confirm.

Do not use if the bolt closes on a field gauge. Look here for headspace info

Quoted: If you put a new bolt in a new barrel you should be fine. It is always a good idea to check the headspace, though. I have seen an AR blow up (not too badly, no damage to operator) due to improper headspace.

Can you describe this in more detail?  How do you know the headspace was incorrect on the rifle in question?  What was the failure?  Ammo type?  What cause the excessive headpsace?  Bad bolt?  Improper barrel extension install?

Quoted: Quoted: If you put a new bolt in a new barrel you should be fine. It is always a good idea to check the headspace, though. I have seen an AR blow up (not too badly, no damage to operator) due to improper headspace. Can you describe this in more detail?  How do you know the headspace was incorrect on the rifle in question?  What was the failure?  Ammo type?  What cause the excessive headpsace?  Bad bolt?  Improper barrel extension install?

It happened to "9supercomp", I'm sure he could give you all the exact info...

Headspace problems with AR's that are in any reasonable condition are about as common as unicorns.

Most kabooms are from something other than headspace... case head separation due to bad brass, plugged bore, etc.

The preoccupation about headspace comes from the old gunsmith's warning, "always check headspace on military rifles".  This was because after WWI and WWII great numbers of surplus military rifles, many of questionable condition, came into the US.

You don't see Joe Nimrod Deerhunter checking his headspace every season, do you?

You can buy an upper from any of a number of reputable outfits, and fit it with a bolt from any other and you will have no problems.

The military armorers do check headspace at regular intervals, but rifles in basic training, or other uses, might or might not be very high milage.  Even if it had excessive headspace, it is still unlikely to cause a kaboom.

The caution about not swapping bolts is that each bolt and barrel break in to a unique wear pattern.  If bolts were swapped, a new wear pattern would have to be established, and wear would be accellerated.  Still, not a big deal.  The rifle is not going to blow up.  If it was a problem, there would be gazillions of dead GI's.

(I'd also like 9supercomp to tell us all about it.)