One of the biggest differences in price when choosing between cheap and high-end Mil-Spec rifles, is the quality of the barrel. Most barrels are made either of chrome molly steel with a chrome-lined bore and chamber, or stainless steel, which typically does not have chrome-lining in the bore or chamber. But either way, if the bore (barrel) has chrome-lining, so will the chamber. M16 and M4 Mil-Spec barrels are made of chrome molly steel with chrome-lining.
Image above - remove the barrel's flash eliminator, you will see a dull grey ring around the muzzle of the bore on a chrome-lined barrel.
Advantages of Chrome-Lined Barrels
Corrosion Resistance - one of the big reasons the military prefers chrome-lined barrels is because they are far less prone to rust when neglected. Rust within a rifle barrel will cause a stoppage, as the round cannot be extracted. However, all barrels should always be well maintained and lightly oiled, even during arduous field operations.
Extended Barrel Life - extremely high temperatures are created within the barrel caused by the hot gasses that the powder creates, and the friction of the bullet travelling down the bore. These high temperatures erode the barrel and reduce its lifespan. The higher the velocity of the round, or the faster the rifle fires in quick succession (burst or fully automatic), the more erosion takes place. This is because the barrel gets so hot that it overheats, and the rifling is eaten away. Chrome-lining protects the bore with a thin coat of heat and pressure resistant chrome, which greatly extends the barrels life by preventing damage to the throat and rifling, specifically when fired in fully-automatic or rapid fire semi-automatic modes. This is the biggest reason the military uses chrome-lined barrels. Most people however will never have the issue of "shooting out" a barrel of any kind.
Cleaning - chrome-lined barrels are easier and quicker to clean than steel barrels, but they still do require daily cleaning when fired as they still foul the same as steel barrels.
Chrome-lined Chambers
Disadvantages of Chrome-Lined Barrels
Long-Range Accuracy - when a barrel is chrome-lined, even with the best modern technology, it is not perfectly even on the inside. This causes a reduction in accuracy when compared to steel barrels, but to most shooters this is not noticeable. If you are building a precision rifle, then a moly-coated barrel is not the best choice.
Cost - chrome-lined barrels take longer to make, use more expensive metals, and are manufactured to higher tolerances than traditional steel barrels. They are therefore more expensive and often the reason for the large difference in cost.
Which Barrel is Right for You?
Most shooters will get by just fine with either choice, as long as you are aware of the advantages and disadvantages of each. When choosing whether to get a chrome-lined barrel or not, budget is often the deciding factor. A high quality AR-15 platform can still be built with a non chrome-lined barrel. The upfront savings can often outweigh the cost of re-barreling your rifle in the future, should you need to. But most firers will never in their lifetime be able to wear out a steel barrel.
If rapid firing and tactical training is your objective, then go with a moly-coated barrel if you can afford to do so. Dumping three or four magazines through a stainless steel barreled rifle without giving it time to cool down can significantly reduce the barrel life, but a good non chrome-lined barrel can last for over 5,000 rounds before it begins to show a loss of accuracy.
If you are building a match rifle that will be used solely for competitions where precision is of great concern, then stick to a chrome-moly or stainless steel match grade barrel without chrome-lining.
