I'm kicking this question to you guys.I would like to either replace my barrel,or put a new upper on my baby.My question is if I just bought a barrel assembly,handguards,gas tube.Could I install this on my current upper,using my current B/BC assembly?I understand headspacing is critical.If I was to buy a factory headspaced barrel,would their still be a problem,with using my B/BC group.If so could I have my new barrel headspaced,for my current B/BC?Also could or is it possible,to have one B/BC group headspaced.so that it would function with my new as well as current barrel?I really appreciate any help you guys could give.I love my guns and shooting.This site seems to be the only place where my love of shootin,collecting,building and sharing common intrest.Thanks again!

In general, I should be able to take any in spec barrel assembly and any in spec bolt and have proper headspace.  You cannot change the headspace on an AR15 except by swapping barrel and/or bolt until you find a set that gives you proper headspace.

If you are worried, buy some headspace guages and pay particular attention to the GO and FIELD guages.  I'm not sure that a SAAMI NO-GO is relevant to a military weapon.

Failure to close on the GO means headspace is too tight and is generally a more dangerous condition as it may allow firing without the action being locked.

Closing on a FIELD means headspace is way to big and will generally result in headspace separations and much gas flowing back into the upper.