All non-stainless AR barrels are made with chrome-moly steel, either 4140 or 4150.  4140 is probably the most common barrel steel used for rifles, but the military requires 4150 steel, which is more expensive and more difficult to machine, but retains its structrual strength at higher temperatures.  This allows hot barrels to remain accurate longer, and reduces the possibiliy of a barrel bursting when subjected to continuous full-auto fire.

Both 4140 and 4150 can be chrome-lined with industrial hard-chrome.  Some manufacturers skip the chrome to save money, and will try to tell you that a non-chromed barrel is more accurate.  The truth is that most barrels made today are amazingly accurate, but not truly match quality, and the accuracy difference between a chromed and non-chromed bore is about zero today.  Perhaps 40 years ago there were problems applying the chrome evenly, but today's barrels show that not to be the case.  To get a more accurate barrel, you need to start with a higher-end barrel blank that's been machined with tighter tolerances, and that's going to cost more money.

-Troy