Here's more on hard chrome lined, stainless, and standard carbon barrels.
Standard carbon steel unlined barrels:
High quality barrels MAY BE slightly more accurate then hard chrome lined. This is not always the case and the difference is slight.
They're the cheapest barrels.
May benefit from a break in procedure.
They rust and corrode if not cared for.
They're the hardest barrels to get clean.
They don't last as long as the others.
Don't get taken by people advertising "Chrome Moly" barrels. Chrome moly is nothing more then ordinary carbon steel used in barrels since the 1930's.
Some sellers leave the impression the bores are lined with chrome by using the "chrome moly" term.
Last longer then plain steel, but not as long as chrome lined.
Easier to clean.
Much less likely to rust or corrode.
For these reasons, virtually all Match rifles use stainless barrels.
Break in is much faster then carbon steel.
Bright finish unless coated or treated to darken it.
The most durable. This is the major reason the military will buy only chrome lined barrels.
Very easy to clean. Fouling won't adhere to the "slick" chrome as it does to carbon. Another reason the military buys it.
Highly resistant to rust or corrosion. The second major reason the military only buys chrome lined.
Requires no break in, and in fact, no break in is possible.
Tolerates high heat from rapid fire much better then stainless or plain carbon steel.
Often not quite as accurate as carbon or stainless. This is only an issue in accuracy Match type barrels where you're trying to get the last bit of accuracy out of the rifle.
In an ordinary rifle the average owner will not be able to detect any difference.
Costs more than carbon.
Bottom line: In a non-Match rifle like a standard AR, given the lifetime of the rifle versus the extra cost for hard chrome, it's sort of silly not to buy hard chrome.
The advantages far out-weight the extra cost.