Keeping hot barrels accurate
Anyone who has spent time at the range shooting high-powered rifles knows that sustained fire or continuously shooting without letting the barrel cool down between strings of shots will get the barrel so hot that accuracy will suffer and shots can be thrown off a couple of inches. It isn’t hard to fire a military-style rifle in semi-auto mode at 100 rounds a minute, and many owners of these rifles do just that at times when shooting at the range.
Even when shooting a semi-auto rifle in rapid fire mode, an enormous amount of heat is generated, which can quickly ruin a rifle barrel .
The leade, which is the unrifled portion of the barrel just forward of the chamber as well as the first few inches of rifling, are subjected to enormous temperatures approaching those on the surface of the sun as well as pressures exceeding 50,000 PSI during rapid-fire exercises.
During slow-fire conditions, this area is allowed to cool sufficiently 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.
Currently there are four very different methods used by shooters to protect and extend the service life of their barrels.
Those who participate in bench-rest and long-range completion use very heavy and long barrels which can last much longer than the barrels found on most sporting and hunting rifles. Still, those big heavy barrels have to be replaced when accuracy begins to deteriorate.
Currently, the military hard chrome lines the barrels of their rifles to protect them from the excess erosion that occurs during sustained fire. This greatly extends the barrel life of rifles that are fired for prolonged periods in full auto or semi-auto mode. It takes a very knowledgeable professional person to evenly apply the hard chrome lining to the inside of a barrel, but the barrel will have approximately twice the service life of an unprotected barrel. Both the military and civilians who shoot semi-auto versions of military style rifles swear by the hard chrome lining of the barrels.
Some claim that hard chrome lined barrels aren’t as accurate as unprotected barrels because the rifling of hard chrome lined barrels is not as sharp as in unprotected barrels. This is true, but the difference in accuracy will never be notice by the majority of shooters. One MOA is pretty common in most of the military rifles and their semi-auto counterparts being built with hard chrome lined barrels today.
Another method of dealing with the heat and pressure that rifle barrels can be subjected to is a process where the un-blued barrel is immersed in a very hot liquid nitride salt bath for a period of time. The process is known as “ferritic nitrocarburizing.” This is not a new technology but has recently been applied to rifle barrels to protect them from the heat and pressure from sustained fire.
Most people will recognize terms like Melonite, Tennifer, Ni-Corr, Blacknitride or Salt Bath Nitride. They are all variations of the same process. During the process, a two-part surface layer is formed, an outer iron nitride layer with a nitrogen diffusion layer below it. Nitrogen and carbon are diffused into the surface of the metal.
Nitriding is not a coating; carbon and nitride are broken down to become a 20 microns thick part of the barrel which stiffens the outside and inside of the barrel but not the core. It creates a hard, slick, friction-free surface that reduces heat and stands up to the high pressures generated by prolonged and sustained fire. Nitride treated barrels have a Vickers hardness of 800 to 1500 HV, which is harder than hard chrome.
One half MOA can be achieved with nitride treated barrels as the rifling remains sharp. Many of the Bench rest and long distance competitors are buying nitride treated barrels for their rifles. Barrels can be purchased for $89 to $100, and several manufacturers such as Sig Saur are offering their AR platform rifles with nitride-treated barrels.
Lastly, two companies — Advanced Barrel Systems and Proof Research — have been trying to wrap carbon fiber around a thin steel barrel for several years in an effort to create a barrel that is accurate and lighter in weight than conventional barrels. The basic idea is that a thin steel barrel wrapped in a tight carbon fiber sleeve would be lighter, impervious to moisture and more accurate due to the stiffness the carbon fiber wrap would produce. The idea has merit, but the science didn’t support the concept at first. Steel barrels expand as they heat up while the carbon fiber used in the original attempts, contracted causing uneven tension on the barrel which adversely affected accuracy.
It was later determined that the wrong carbon fiber was being used. Advanced Barrel Systems came up with a carbon fiber that is the result of the polymerization of coal tar pitch. Tests so far indicate that the tar pitch fiber actually does conduct heat away from the barrel and accuracy is much improved, while the carbon fiber keeps the barrel stiff without producing uneven tension. I plan to watch this technology for the next few years. I don’t think they are through improving the process yet.
Smokey Merkley was raised in Idaho and has been hunting since he was 10 years old. He was a member of the faculty of Texas A&M University for 25 years. There he taught orienteering, marksmanship, self-defense, fencing, scuba diving and boxing. He was among the first DPS-certified Texas Concealed Handgun Instructors. He can be contacted at email@example.com.