For a long time, everyone and their brother wanted a chrome-lined barrel. They’ll pay extra for it. They’ve heard it’ll last almost forever. They’ve heard it’s the best barrel that ever existed.
And they’re only partly correct.
Chrome lining can protect the barrel from heat and pressure. That’s true.
A good chrome lined barrel can still last for 5,000 rounds before you see any degradation. Maybe more.
However, chrome lining can be uneven in barrels, depending on the manufacturer. The barrels can be SLIGHTLY less accurate than a non-lined barrel (you ARE lining the rifling inside the barrel and therefore there is a slight coating over the lands and grooves, dulling them ever-so-slightly.)
Essentially, chrome-lining is one of those leftover processes that “a guy at the range heard from his buddy at work that his dad always told him…” that chrome-lining was the ONLY kind of barrel you’ll want/need.
This is not to take away from chrome-lined barrels. Like I said, they protect against heat and pressure in an AR. It’s not to say that there are zero benefits to using one.
But there are a LOT of other choices when it comes to barrel treatments. See also Meloniting, Gas Bed Carborization, and Cryogenic Tempering