AR barrels are usually made from 4100-grade Chrome Moly steel alloy, most commonly 4140 -- which can be cheaper -- or 4150 which includes Vanadium in the alloy to increase strength.
The hardness ratings of each type are the same, and while each alloy does include chrome, it is not a large enough amount eliminate rust forever.
416R Stainless Steel is also commonly used in AR barrels and it has more chrome, making it more corrosion resistant.
It can be polished for a distinct look, shot peened for a matte, less vibrant appearance, as well as colored with chemical treatments to feature a dark finish.
Chrome lining in the bore of a barrel was seen as a huge breakthrough for reliability when it was first introduced.
The lining provides a protective coating against corrosion and fouling, which increases the life of the barrel. However, over time, chrome lining can reduce accuracy since the coating unevely wears down the length of the bore.
Many manufacturers have refined their processes and produce barrels that are highly capable with chrome lining.
More recently, proprietary methods of nitrocarburizing the lining of a barrel have been patented, and these forms of barrel linings will use trademarked names like Melonite, Tenifer, Salt Bath Nitride, and others.
The nitrocarburizing has the outomce of a chemically altered surface inside the bore of the barrel. This lining is becoming more popular, due to its lack of negative effect on accuracy.
For the most supreme accuracy, an unlined stainless-steel barrel will give the best performance. The tradeoff is a lack of protection against corrosion, and shorter barrel life.
Those who are looking for a barrel that will perform at a high level across the widest number of applications, a 4150 Chrome Moly Vanadium barrel with a chrome or nitrocarburized bore lining will fit the bill.
If you’re building a precision rifle that needs ½ MOA accuracy and will never be subjected to sustained high rates of fire, a stainless steel, unlined barrel may be what you’re looking for.
Some of you might be thinking to yourselves “just tell me which one’s MIL SPEC!”, and that’s fair. The AR-15 in a MIL SPEC configuration is a fine firearm. If you were looking for a barrel to attach to a MIL SPEC rifle, you’d be after a 4150 Chrome Moly Vanadium barrel that is phosphate coated and chrome lined; like this upper from BCM.