Ive got a 20" heavy chrome lined barrel that I am going to mate to a flat top upper so I can use a scope. Its going to be my third gun when done and I am going to put a bipod on it as well so it will be a target rifle.
I have read a lot of stuff about using other bolts and carriers, is it ok to share a bolt and carrier? My other two rifles are very new with only a couple hundred rounds fired. Can I use the Bolt and carriers, swap them or not?
Some people say yes,others say no,and the mil manual says its dangerous.
Also is it true that headspace is set at the factory in the barrel...Its a Bushy barrel and their catalog says they are head spaced.
The other uppers I have where complete that I have on my two other guns.

Generally, there is no problem sharing bolt/carrier assemblies with multiple barrels.  

The important issue is that your single bolt and each other barrel you use it in have the correct headspace.  As long as your bolt and the mating barrel assemblies are within their respective drawing specifications you should not have any problems.  

Always be safe and have the headspace checked by a competent gunsmith or educate yourself on which components determine and affect headspace so you can check it yourself with the proper gages.

It should be noted that the bolt carrier, by itself, has zero affect on headspace.  As the name implies, it only "carries" the bolt.

Barrels are set for headspacing at the factory with their bolts.  It is certainly possible to use other bolts and swap back and forth, but it isn't recommended.  It isn't worth screwing up any of my AR15s, I always dedicate a bolt & BC for each upper.  The extra hundred bucks is well worth it.  I can't see the point in sharing BCs and swapping bolts.

Remember that your barrel and bolt are going to wear, when kept together they will wear as a unit.  If swapped back and forth, they will not.

Is it worth the extra couple bucks to you?

Point taken guys, whats another $110 for a bolt and carrier when i have so much money invested in my ar's already....I'LL get a complete assembly next month when I have some spare cash!!!!!
I mated the flat top and barrel last night and all went well...Everything is straight and went into place.

Knowing the above information, why do uppers in the EE almost always come without b/c/ch?