September 23, 2010
By J. Guthrie
A new angle on wrapping a rifle barrel in carbon fiber promises lighter rifles and greatly increased barrel life without sacrificing accuracy. In some cases, it might even improve accuracy. Here's the scoop.
By J. Guthrie
First hoops are wrapped to form a base layer (top) and then a helical wrap is added (center)--its angle changing with caliber and bullet weight. After a resign is applied and baked to bond the elements together, the barrel is rough ground to the right dimensions. Then a fine grinding applies the smooth end finish (bottom).
Rifle barrels wrapped in a sleeve of carbon fiber were a revelation to shooters looking for heavy-barrel performance from a featherweight rifle. The miles of carbon-fiber filament that wrap a barrel liner weigh five times less than the same amount of steel. Stiff and lightweight, the barrels deliver more with less.
While these barrels--paired most often with hunting rifles--offered shooters performance gains, there are those who sought to gain more, looking for carbon fibers to provide more than a dramatic reduction in weight. Mike Degerness, president of Advanced Barrel Systems (ABS), is one of those guys.
Degerness has been using new technologies and different kinds of carbon fibers to not only lighten rifle barrels but more importantly to increase accuracy and longevity. The result of a decade's worth of experimentation is a carbon-fiber wrap that is so effective at managing heat--a barrel's number one enemy--it is being applied to military weapons, including belt-fed machine guns.
Like many others who developed groundbreaking firearm technologies, Degerness did not start off in the firearms business. In fact, his career in composites came after he retired from a 20-year run as a law enforcement officer and paramedic.
"Lincoln, Nebraska, is a hot bed of composites research," Degerness said. "I went to work with a small composites company that produced golf-club shafts, launch tubes for missile systems, and antennae masts for humvees."
Missile systems and golf clubs were fun, but to Degerness, they were not nearly as exciting as guns, especially for a man that had a passion for precision rifles. Before Degerness hit his teenage years, his father and Uncle John (John Anderson, editor of The Varmint Hunter Magazine) had him set up with a rifle and an education on the finer points of reloading, ballistics, and hunting. It was only natural that a few decades later, Degerness applied his knowledge of composites to rifles.
"We wanted to make a barrel more durable than lightweight, and that would be accomplished by reducing the heat generated by firing," Degerness said. "It was a long road to get to where we are with the barrels."
Most companies use polyacrylonitrile (PAN) carbon fibers exclusively to wrap barrels. The PAN fibers are petroleum based, a derivative of crude oil. They have exceptional tensile strength, and when wrapped around a liner at opposing angles and set in a hard resin, they provide an extremely rigid structure. According to Degerness, the catch was that his extensive testing proved this type of fiber conducts heat only marginally better than standard barrel steel.
"Carbon fiber conducts heat like fiber optics conduct light," Degerness said. "They have very little ability to transfer heat from fiber to fiber. Most wrapped barrels have fibers running generally parallel to the axis of the bore, and that limits the ability of the fiber to dissipate heat."
Barrel steel, depending on the alloy, has a thermal conductivity coefficient of around 21 W/(mxK), and PAN fibers have a coefficient of 19 W/(mxK), so they are not much help conducting heat away from the bore and radiating it out into space. The key to making the concept work would be finding a carbon fiber that conducted heat.
Degerness found his miracle material, pitch-based carbon fibers, on the race track. Made from coal, pitch fibers were used for Formula One racing brake pads. The material was also used in the space shuttle's skin for dissipating the extreme heat generated when reentering the Earth's atmosphere and moving it to noncritical areas. But there was a big catch.
ABS uses a liner that can be fired without the wrap; it has thick profiles at the chamber and muzzle that guarantee safety.
"Pitch fibers have a coefficient of thermal conductivity of over 700," Degerness said. "But the major challenge is that the higher the coefficient, the more it shrinks when heat is applied. As metal heats, it expands. You have to minimize this (in barrel manufacturing), or you will get a wandering point of impact. Getting the two to work together was tough."
Early barrel wraps spun entirely from pitch fibers were a disaster.
"You could shoot a .300 Win. Mag. five times, and the wrap would shrink 1/8 inch back from the muzzle nut," Degerness said. "The gun would go from minute of angle to shooting 6-inch groups at 100 yards."
His answer was a matrix of PAN fibers and an epoxy mixed with small, chopped sections of pitch fibers. Because they have a thermal coefficient similar to barrel steel, the PAN fibers provided an extremely rigid structure that changed very little when heated and also dampened the barrel's harmonic signature. The chopped pitch fibers connected the long PAN fibers in the wrap and helped conduct heat away from the bore at an amazing rate. Degerness had found a way to get the best of both worlds.
"The resin matrix that holds everything together is a key component," Degerness said. "The epoxy we are using is one of the few that will tolerate heat up to 500 degrees Fahrenheit before it starts to break down. There are a lot of weapon systems that get much hotter than 500 degrees, but the pitch fibers cool so fast that the matrix never reaches that temperature."
ABS had its eye on several military applications, so it sent several barrels out for third-party testing. The results were astounding. A composites lab at the University of Nebraska found that the carbon fiber-wrapped barrel cooled 372 percent faster than a steel bull barrel with the same profile.
"Watching M4 barrels through thermal imaging goggles is just amazing," Degerness said. "On full auto, you see a starburst of heat come off the barrels, and in seconds it's back down to ambient."
This greatly increases barrel life. For crew-se
rved and full-auto weapons, this means higher rates of sustained fire before barrel changes or burnout. Degerness has one customer who uses a wrapped barrel on an M4 to demonstrate a suppressor system. The rifle's barrel, made from a vicious 15-5PH stainless steel, had 46,000 rounds of full-auto, suppressed fire through it before the gas system finally blew up.
For varmint and big-game hunters and competitors, it means barrels that maintain their peak accuracy exponentially longer than steel barrels. Bolt-gun shooters simply cannot fire the rifle fast enough to stress the barrel before it cools. One ABS client did extensive field-testing before deciding to equip its sniper teams with carbon-fiber-wrapped barrels. The client sent out two batches of rifles, all chambered for .300 Win. Mag., with its teams. One set of rifles had Rock 5R stainless-steel, fluted barrels, and the other had Rock 5R barrel liners with a wrap. All barrels had a standard M24 contour. The accuracy standard was 0.6 MOA. Standard barrels were retired because they failed to meet accuracy standards after an average of 1,400 to 1,500 rounds. Composite-wrapped barrels had an average service life of 4,600 rounds and were only replaced then because incredulous armorers were getting nervous.
Though the wrap's primary goal is to cool the barrel, it also reduces weight. This bull barrel weighs 20 percent less than an all-steel counterpart.
"In seven years, we have yet to have a barrel come back for a re-bore," Degerness said. "The initial expense of an ABS barrel is higher than the cost of installing other custom barrels. But when you consider their extremely long service life, you come out cheaper in the long run. If you do manage to wear one out, we will re-bore and re-rifle to the next appropriate caliber for $150."
Another advantage of the wrap is its effect on the barrel's harmonic signature. Barrels, like a tuning fork, vibrate when fired. The carbon-fiber wrap acts as a mechanical interrupter that limits vibrations. The fiber's high modulus of elasticity lowers the amplitude of the vibrations and increases the frequency, thus it has the theoretical ability to improve accuracy. Old benchrest shooters used short, fat barrels to achieve the same effect, often cutting off the last 1/4 or 1/8 inch and recrowning in an attempt to tune the rifle to a particular load.
"Our barrels don't have to be short and fat," Degerness said. "Around the Remington varmint contour, we reach a point of diminishing returns in regard to accuracy."
ABS wraps start with an 89-degree wrap against the steel liner, and then the fibers are added in a helical fashion. The wrap angle is higher at the breech end and lower out towards the muzzle. Additional hoops, perpendicular to the axis of the bore, are added to help linear stabilization. Helical wraps and hoops are added until the desired barrel profile is achieved.
"Figuring out the wrap angles was critical, and it was all trial and error," Degerness said. "We would try different angles, work up a barrel, and go out and shoot it. Over the years, we've come up with some constants as they relate to contour and caliber. We can match the wrap to the twist rate and grain weight of the bullet."
Degerness is quick to point out that inherent accuracy starts with the barrel liner and ends with the gunsmith who installs the barrel. He claims varmint and tactical rifles assembled by top-shelf gunsmiths will turn in 1/4-MOA groups, and big-game calibers usually achieve 1/2-MOA accuracy. The process can be applied to a factory barrel, but the company prefers to start with a premium liner from a specialty barrel manufacturer. While ABS accepts barrel liners from a half-dozen different manufacturers, 90 percent of them come from Satern Custom Machining in Estherville, Iowa. The five-groove cut rifling is silky smooth. If blanks do not meet ABS specs, they are hand lapped to get them right.
An ABS liner is much thicker than those used by other companies. Traditionally, companies looking for the lightest possible weight may not be safe to fire without the carbon-fiber wrap. ABS liners have, at a minimum, a No. 1 profile but can change depending on the end user's caliber. A 22-inch barrel with a No. 7 contour weighs 2.5 pounds.
"The carbon wrap is there for thermal dissipation and harmonic dampening, not to make the barrel lighter, though there is usually a decrease in total weight," Degerness said. "It's a high-tech aerospace application, but at the end of the day, it's string and glue. Generally, the thickness at the muzzle is 0.535 inch, and the thickness at the barrel's peak-pressure point is 0.8 inch, so it is safe to fire our liners without the wrap."
Mike's 12-year-old son Jon Degerness built this custom .243 rifle for a school project and then hit a prairie dog at 672 yards with his rifle.
Before the wrapping process, liners are bore scoped, mechanically gauged, verified for twist rate, and weighed. A CNC filament-winding machine--basically a lathe that applies material instead of taking it away--adds the carbon-fiber filaments. The resin is applied to the barrel and cured in an oven. A rough grinding unit removes the excess stock, and then a CNC machine grinds the barrel to the final profile.
"We know exactly what the barrel should weigh after the wrapping process, and if it's too light, it points to voids in the matrix," Degerness said. "If we find voids, the wrap is stripped off, and we start from scratch."
Some mechanical bonding features are machined into the liner, and the muzzle nut is permanently bonded using an engineering-grade ceramic adhesive. Over the last seven years, Degerness said just four wraps have worked loose, but accuracy was unaffected. The composite wrap is very durable--it takes diamond tooling to machine it--and stands up well to heavy hunting and shooting wear and tear.
"If you end up using the barrel for a baseball bat, the worst that will happen is a small chip," Degerness said. "The top layer is there for looks and has no effect on the structural system. You can buff out any scratches or dents with an Emery cloth, and the pitch fibers limit any cracks from running."
The wrapping process takes about four days, but barrels are done in batches to increase efficiency. ABS can usually turn around a bolt-gun barrel in 30 days, while gas guns take a few weeks more. The wrap can be applied to any gun in almost any caliber. The company has wrapped barrels in .17 Mach Four and helped a customer produce a 10-pound .50 BMG bolt rifle. Wrapping a customer-provided blank runs from $600 to $900.
Degerness was very candid about the practical applications of thi
s technology to the "art of the rifle." He suggested that a hunter looking for two or three shots on a hunting trip from an ultralight rifle would be better served with a conventional super-light contour. If the rifleman is expecting the rifle to deliver extreme accuracy shot after shot after shot on the battlefield or in a dog town, the ABS wrap is probably the way to go.