A common question about AR-15 barrels is Chrome Lined barrels vs the newer Nitride barrels. Which is better? What are the pro and cons of each? The answer depends mostly on what the barrel is going to be used for.
The standard M16 or M4 barrel is lined with chrome and has been since the 1960's. Weapons with full auto capability would wear out a barrel quite quickly with a bare metal barrel. Chome has good wear resistance and it prolongs the life of the barrel especially when subject to the abuse of full auto fire. An added benefit is chromes excellent corrosion resistance and it makes the barrel a bit easier to clean. When a barrel is Chrome Lined it adds a thin layer of Chrome to the bore of the barrel. The lining technically can reduce the accuracy of the barrel a slight bit. The lining also can wear off over time with use. The outside of the barrel is usually given a black phosphate coating to protect from corrosion.