Borescopes Target Rifle Barrel Defects

As one of only three manufacturers to receive a contract to produce mil-spec M-16 rifles, Sabre Defence must produce barrels to the most stringent quality standards. Hawkeye Borescopes from Gradient Lens Corporation offer the only way to view the insides of the barrels and meet the company’s visual inspection standards.


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Employees at Sabre Defence Industries, a firearm manufacturer in Nashville, Tennessee, are used to working under pressure. In addition to tight production deadlines, the company has to contend with the fact that its largest customer can reject an entire production lot based on one out-of-spec feature in a single part. That’s why the ISO 9001-2008-registered shop has implemented an especially demanding quality process to ensure that all products are produced to the most stringent specifications. Key to these efforts are borescopes from Gradient Lens Corporation (Rochester, New York) that provide the only reliable means to visually inspect the insides of rifle barrels.

The customer mentioned above is the U.S. government—in 2008, Sabre became the third manufacturer in history to receive a contract for military-specified M-16 rifles. This achievement didn’t come easily. The company had to establish that it was capable of producing both the quality and the quantity the military requires. "We had to go through an extensive first-article process, which included endurance testing and accuracy testing before and after the endurance tests, just to mention a couple of the requirements" says Morris Blanton, the company’s quality manager. "Compared to the commercial market, where you can use, say, a lighter or cheaper material, we don’t have the same options."

However, the company’s drive for quality leads it to apply the same strict standards to its commercial jobs, such as AR-15 rifles that are used in law enforcement, competition shooting, hunting and more. Other parts produced at the 45,000-square-foot facility over the years include components for the Army’s M-2 Browning .50-caliber machine gun and M-60 machine gun, the Air Force’s M-134 7.62-mm minigun and the AUG rifle for Austrian-based Steyr Mannlicher. With just over 100 employees, the company is relatively small compared to the other manufacturers with M16 contracts, Mr. Blanton says. Processes range from basic milling and turning to deep-hole drilling and cut-and-button rifling.

Quality assurance permeates all areas of the operation, with both scheduled and random inspections performed at all stages of the manufacturing process from receipt of raw material to the finished product. This is essential to ensure that each component provides the specified fit, finish and performance. Although the company’s standard measuring equipment, CMM and air gage provide the best means of conducting dimensional inspections, none is capable of detecting internal barrel defects.

Years ago, inspectors would simply look through the barrel with the naked eye, and any noticeable defects would appear as a shadow or dark spot. This posed a number of problems: the risk of not identifying a defect at all, an inability to determine the nature of a defect when spotted, and a great deal of time spent cleaning barrels. "The .50-caliber barrel is 45 inches long, and if there’s a defect in the middle of it all you’re going to see is a dark spot. That might be just a spot of grease, so to be sure, you’d have to clean a barrel several times until it was spotless."

Now, Sabre uses rigid borescopes from Gradient Lens Corporation’s Hawkeye Pro line in various lengths and diameters. The core technology of these scopes’ design is the manufacturer’s proprietary endoGRINs relay lens system, which provides high image quality at a lower price than comparable models, says Doug Kindred, president of Gradient Lens.

Many borescope relay systems, which link the eyepiece on one end to the objective lens on the other, consist of a series of pairs of tiny, curved lenses. Scopes typically use fiber optics connected to a light source to illuminate the interior surface to be inspected. Like a refracting telescope, the lenses in the relay system focus and refocus the light along the length of the device to form an image that is magnified at the eyepiece. These lenses vary in size according to the diameter of the borescope, but all are small—ranging from 0.5 mm to about 4 mm in diameter—and must be manufactured to exacting precision. This can be a daunting, time-consuming process that involves grinding, polishing, coating and centering large numbers of the tiny lens pairs for each scope.

In contrast, the endoGRINs relay system incorporates rods of chemically treated glass, which is known as gradient-index, or GRIN. Unlike the lenses mentioned above, a gradient-index rod bends light continuously along its entire length. "The middle of the rod has a high refractive index, and the outside edge has a low refractive index," Dr. Kindred explains. "Rather than curved surfaces, you’re using the gradient in the material to focus the light." He notes that the advantage of this system is twofold: the rods are much easier to manufacture than the lenses used in other scopes, and fewer are needed to produce a quality image. "That gives us a huge cost advantage over the competition."

At Sabre, the scopes are employed at a number of stages in the manufacturing process that can cause unwanted defects, namely before and after chrome plating and during the rifling and chambering processes. Barrels are chrome plated to increase durability and barrel life, reduce erosion and corrosion, withstand heat and ease cleaning. However, if the chrome doesn’t adhere well to the surface, it can flake off and become porous. Scoping the barrels after chrome plating enables inspectors to spot these flaws, while scoping before that process enables the detection of machining defects or loose debris that contribute to their occurrence in the first place, Mr. Blanton says.

Chambering refers to the operation that creates the chamber, which is the portion of the barrel that houses the cartridge prior to firing. The type of defect that can occur here depends on the chambering method, which can range from forging to drilling and reaming, Mr. Blanton says. "Depending on the process, we’ll look for heavy rings or lines, corners breaking down along the angles in the chamber, and other such issues," he explains. "For example, heavy rings around the inside of the chamber can allow the brass casing to swell and bulge into those rings, causing extraction problems with the shell casing anddamage to the brass if you’re a reloader."

Once completed, barrels undergo another round of scoping in the quality lab and yet another round after test firing. Here, operators look for flaking chrome as well as throat erosion in the chamber, where most of the pressure and heat accumulate during use, Mr. Blanton says. Any visible erosion gives the company an idea of the longevity of the materials it uses, which include durable metals such as Chrome Moly-Vanadium alloy for the majority of its barrels and 410 stainless steel for a few commercial models.

Sabre inspections also make use of a borescope video system with a live camera that displays images on a computer monitor. The scopes themselves magnify an image by a factor of 10; with the camera system, this is increased threefold to 30-times magnification. This not only helps operators spot even the smallest defects, but also aids in training new employees because trainers can easily point out specifically what to look for when scoping a barrel. Moreover, the video system prevents the inevitable eye strain and discomfort that would result from long hours spent inspecting hundreds of barrels, Mr. Blanton says. "Overall, I would say that the scopes have reduced the possibility of sending out a barrel, whether commercial or government, that might have a defect inside it," he concludes.


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