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.
Nitride is a treatment to the barrels steel that hardens the steel and significantly increases corrosion and wear resistance. This is usually done in a process called Quench polish quench (QPQ). Melonite is a specific version of Nitride but for the most part Melonite and Nitride are the same. Since it isn't a lining accuracy is not degraded and it cannot be worn off. The increased wear resistance means the barrel can hold up over time to normal or even rapid semi auto fire. Nitride treated barrels are cheaper to produce than Chome lined barrels and easier to clean.
Stainless Steel barrels have long been used for their better accuracy over Chrome barrels, but suffer from shorter barrel life. Stainless Steel can also been treated with Nitride to improve the wear resistance and improve barrel life without hurting the accuracy.
The YouTube channel Mrgunsngear did a full auto test with two identical 4150 steel Faxon barrels, one Chrome Lined and the other Nitride. The results were interesting and I won't give away the entire test results here. But the biggest take away from the test is the Nitride barrel held its accuracy better.
So which is better? Since most people don't have full auto receivers they won't need the better heat resistance that Chrome offers. With Nitride being cheaper and the potential for better wear resistance over time when using semi auto fire; Nitride would be the way to go on a semi auto weapon.