Time to show my ignorance.  I have built a lower and I was planning to buy an upper.  From what I'm reading, a chrome-lined barrel is the way to go.  So when I started seeing barrels described as "chrome-moly" I thought that was the same thing.  Then I found a page selling uppers that specifically described their uppers as "Chrome-Moly NOT Chrome-Lined."  

So, before I spend my money what's the difference?  And should I be particularly worried about it?

Never mind.  I just answered my own question.

http://groups.msn.com/ThePennsylvaniaAR15ShootersSite/barrelfaq.msnw

Hey Brian,   I have a colt.  Do you know is mine chrome lined?  The barrel reads "c mp 5.56 nato 1/7.  I know what the rest of the stuff is but I'm trying to find out if my barrel is chromed in my manual and I'll be darned if I can't tell.  It's a "sporter target model"  basically government style.

I have no idea.  I suspect Colt could tell you, but I couldn't.

All of Colt's military profile/twist barrels are chrome lined, as are there pre-ban barrels.  They only started using non-chromed barrels in the past few years on thier 1/9 twist barrels.  Even then, they care chroming the chamber which helps reliability, just not the bore which would make cleaning a little easier and increase barrel life.

It is safe to assume that anything that says begins with "C MP" and ends in "1/7" is fully chrome lined.

Thanks hoplophile.