http://cheaperthandirt.com/blog/?p=2678
Ever since the military began chrome-lining barrels on standard issue machine guns and rifles there has been debate concerning the benefits and disadvantages of chrome-lined barrels. Many of the benefits of chrome-lining are shrouded in myths and misconceptions. Chrome-lining protects the barrel from corrosion, but this is not the main purpose for lining a barrel. Chrome-lined barrels are also easier to clean, but the military would not invest in a chrome-lined barrel just to save a grunt some time swabbing out the bore.
Years ago, with the introduction of high powered machine guns and semiautomatic rifles capable of sustained high rates of fire, military armorers began to notice significantly increased barrel wear and erosion. Older models of the most powerful machine guns were capable of “shooting out” a barrel in less than 1,000 rounds! Chrome-lining was introduced to increase the barrel life, allowing more rounds to be sent down range in less time without the need to replace the rifle barrel.
 Muzzle flash from an AR-15 rifle, demonstrating the immensely hot gases generated by powder combustion. Photo courtesy of bdjsb7 licensed under Creative Commons. |
Nowadays, almost all military rifles are universally chrome-lined to protect the rifle barrel from excess erosion. AR-15 rifles are particularly prone to erosion when fired rapidly, in part due to the high velocity of the round, and in part due to the high pressures generated by the cartridge. While it’s not uncommon for military rifles to experience high rates of sustained fire, it’s also not difficult to fire a semiautomatic AR-15 at rates exceeding 100 RPM. Under sustained fully automatic gunfire, or rapid semiautomatic fire, an enormous amount of heat is generated. That heat is what can quickly ruin a barrel.
The leade (the unrifled portion of the barrel just forward of the chamber), as well as the first few inches of rifling, is subject to temperatures hotter than the surface of the sun and pressures exceeding 50,000 PSI. Under slow fire conditions this area is able to cool a sufficient amount in between strings of fire. Under sustained rapid fire however, there is no time for the heat to dissipate and temperatures soar into the thousands of degrees Fahrenheit. This can quickly cause damage by eating away at the rifling, “burning up the barrel” with the combination of extremely high heat and pressure. Hard chrome-lining the bore protects the leade and rifling with a thin coat of heat and pressure resistant chrome. This greatly extends barrel life in rifles that are fired for prolonged periods in full-auto or rapid fire semiautomatic modes by preventing damage to the leade and rifling.
There are many people who argue that chrome lined barrels are less accurate than an otherwise identical steel barrel. All things being equal, this is true, but for most shooters, the degree to which accuracy is lost by using a chrome-lined barrel is generally unnoticeable. A chrome-lining does diminish the sharpness of the rifling, but the accuracy loss from this is not insurmountable. Consider the Fabrique Nationale SPR rifle (what is essentially a gussied-up chrome lined Winchester Model 70) is capable of shooting a 1/2 MOA group at ranges up to 800 yards away. A sub MOA gun is a fine rifle by nearly anyone’s standards, and most well built chrome-lined AR rifles are capable of 1/2 MOA groups as well.
So, when should you go chrome lined? Most casual shooters will get by just fine with either chrome-lined or non chrome-lined barrels. The fact is, most of us don’t get out to the range often enough, nor engage in rapid fire when we do (most ranges prohibit the practice). Even shooters who occasionally engage in periodic rapid-fire with their AR-15 style rifle may not notice the effects of excess barrel wear for a number of years. A good non chrome-lined barrel can last for over 5,000 rounds before it begins to show a loss of accuracy. If you shoot 1,000 rounds a year, even blasting through a full magazine as fast as you can pull the trigger on ever range trip, it could take 5 years or more before any significant loss of accuracy begins to become apparent.
When deciding whether to get a chrome-lined barrel your budget may be the deciding factor. Non chrome-lined barrels are significantly less expensive than a similar chrome-lined barrel. For a shooter who wants to build a quality AR-15 with less initial investment, non chrome-lined barrels represent a great cost-saving measure, combining the inherent accuracy of a stainless steel or chrome-moly steel barrel with an acceptable barrel life for a hunting or sporting arm. The up front savings can easily outweigh the cost of rebarreling your AR years in the future.
For the serious shooter who needs maximum barrel life as well as accuracy, a chrome-lined barrel represents the best of both worlds. The accuracy lost from a chrome lining amounts to less than 1/4″ at 100 yards, a negligible amount for most AR rifles used in tactical applications. If you do decide to go with a barrel that is not chrome-lined, be aware that you can significantly reduce the barrel life by quickly dumping 3 or 4 magazines through it without stopping to let it cool down.
If you’re building a match rifle that will be used solely for competitions where you’ll be firing slowly and you need a great deal of precision, stick with a chrome-moly or stainless steel match grade non chrome-lined barrel. If you think you’ll ever want to use your AR-15 for tactical applications, or even just rapid-fire plinking, drop the extra cash and get a chrome-lined barrel. Otherwise, be aware that without a chrome-lined barrel your AR-15 should be allowed to cool between magazines in order to avoid damaging the barrel.
