Since the Ar15  design lends itself to  swapping  uppers depending on the  intended usage of the weapon at any particular time I was wondering   if  the members  who have  switched  uppers on their  AR  use the same  bolt carrier/bolt  in  the   the uppers they  switch on and off  or  do they  use a  separate bolt carier and bolt for each upper.    There are some  prety strong  warnings inthe M16 USMC tech manual against swapping  bolts & carriers. Since I  am considering    doing some upper swapping  I was  curious/concerned about the safety issues

I have been swapping for 12+ years.  I have never had a problem.  I use a head space gauge but I don't worry about it.  I do it less often now that I am blessed to be able to have a carrier for each different upper.  There has to be a reason however for the warnings by the military.  I can't say what is right for any one else just that it has worked for me in the past.
pjc  

Quoted: Since the Ar15  design lends itself to  swapping  uppers depending on the  intended usage of the weapon at any particular time I was wondering   if  the members  who have  switched  uppers on their  AR  use the same  bolt carrier/bolt  in  the   the uppers they  switch on and off  or  do they  use a  separate bolt carier and bolt for each upper.    There are some  prety strong  warnings inthe M16 USMC tech manual against swapping  bolts & carriers. Since I  am considering    doing some upper swapping  I was  curious/concerned about the safety issues

I swap uppers/lowers on occasion, but always keep the bolt groups with their respective uppers.

It's a good practice to at least keep the bolt itself matched to the barrel it has been assembled/headspaced with, the bolt carrier is less important.  

Eidted to add: most likely you would be more prone to encounter a problem swapping a bolt/barrel with a large amount of wear/usage on it than a newer bolt/barrell with little wear.

I swap quite a bit. Have separate bolt carriers for each upper.

The bolt itself headspaces on the the barrel.  You can swap carriers all day long without effecting headspace.  The reason the warnings are there is because say one Military rifle has 25K through it and another has 2 rounds through it.  If you take the old bolt and put it in the new rifle or vise versa you may have excessive headspace which might Kb the rifle.