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Cryogenic Stress Relief For Greater Accuracy

 

Sighting-in a shotgun requires shooting a test pattern on paper to show the difference between the “point of aim” and the “center of pattern.” Rarely would a shooter fire 25 rounds and then re-test. But we did just that at 300 Below, Inc. to find out if barrel heating does affect barrel accuracy.

Shooting causes heat, and heat can cause a barrel to warp due to tensile and compressive metal stress located randomly throughout the barrel. Accuracy is affected. Why? The barrel bends and warps as it heats then returns to the original axis line as it cools. After 25 shots, the point of aim can vary as much as 12″ at 30 yards when compared to actual center of pattern, or the equivalent of as much as a 40% miss area!

Accuracy of a shotgun barrel is normally corrected by sight adjustment. Generally, point of aim and center of pattern don’t differ at 30 yards by much more than an inch or two, but few shooters ever think to try sighting-in again after the barrel is miraging from heat. The bad news: In some barrels there can be a big area of variance. The good news: It can be cured.

Deep Cryogenic Stress Relief
Three shotguns were randomly selected for trial. They were a 15-year-old Smith & Wesson 1000, a 25-year-old Remington 1100 and a Winchester. All three barrels were subjected to bench rest patterning at 30 yards with l shot, 10 shots and 25 shots. The 85% pattern was 30″ in diameter (a 15″ radius). The patterns moved progressively in all three barrels, ranging from a low 3.5″ average to one pattern which moved a whopping 10.5″ from the point of aim. The overall average of the three shotguns moved 5.9″ from the first shot to the 25th shot.

The barrels were then stress relieved using a thermal cycling process called “deep cryogenic tempering” and were shot again. The results were staggering, with a maximum of less than 1.5″ of movement noted in all three barrels. Target-area lost went from over 40% to less than 5%. Trap shooters have noted an immediate gain in averages after such treatment. Three separate shooters with 90 averages saw their scores go to 95s, and all for less than the cost of a day of trapshooting!

The photographs on page 16 and 18 are of a standard patterning paper available from Shootin’ Accessories. These papers are high-quality, have extensive instructions and are made of a durable stock. For this test, we used a Beretta 390 prepared by Jack Concannon with the forcing cones

bored, a Rhino choke (light modified at 0.710) and a mercury stabilizer. The loads used are readily available –Federal 7½s, 1 1/8 ounces of shot with 3 drams of powder. For simplicity, rather than counting pellets we placed a dot on every target “broken” on tile paper and an “X” on any target not broken within tile standard 30″ heavy-lined circle. The patterns were shot at 32 yards, resting on a bench. The sight picture is shown on page 15.

On the first shot, 20 clay targets were left unbroken. The center of the pattern was 6″ from the center of the point of aim. After the heat of 25 rapid shots, only 6 targets were unbroken. This is unusual in our experience: usually the move is away from the point of aim.

Data is also available front benchrest rifle shooters, who measure their groups of shots in thousandths of an inch. Geza Nagy did an independent and unsolicited trial of the deep cryogenic process for his rifle barrel and published the results. He was firing a 6 PPC LV rifle, and prior to treatment 500 rounds were fired. Many of the target groups measured between 0.350 and 0.500 at 100 yards. The barrel was sent to 300 Below, Inc. for cryogenic stress relief and returned for testing.

Five-shot groups were fired into six targets. After the deep cryogenic processing, the average was a very respectable 0.2245 grouping. His findings? “One thing was clear from the start…there had been a tremendous increase in the accuracy of the barrel brought about by the deep freeze alone,” (Precision Shooting. 12/94).

We have treated over 10,000 barrels, some for top national shooters like Walt Berger, Ellis Lea, Sr. and Roy Baumgardner. Virtually every one is a success story. In May 1996 the 1,000-yard international benchrest shooting record was shattered by Bill Shehane at 3.14″ with a cryogenically stress-relieved rifle.

How Does Deep Cryogenic Processing Relieve Stress?
Residual stresses exist in shotgun barrels from the original steel forming and forging operations, along with the many different machining operations needed to finish the barrel. These operations create a complex, invisible, random pattern in the steel. A steel barrel expands from the heat generated by firing, hut stress impedes expansion and steel barrels often warp randomly oft axis as they are heated by sequential firings. Residual stress can cause a barrel to progressively warp an arc as it heats more from each round fired. A title barrel placed in a vise and fired repeatedly will often show a resulting arc of round placement at the target.

No two barrels are the same, but the effect has a memory and will continue with each group of shots fired. The “loose-cannon effect” is measured in microns at the barrel end, but the distance to the target amplifies the problem, causing widespread groups or patterns. The problem is not a crooked barrel but one which moves after repeated firing. Residual stresses are

uneven and are located variously throughout the structure. Deep cryogenic stress relief is an exceptionally effective method for decreasing residual stress and also increases the durability or “wear life” of the steel.

Fold a piece of paper and crease it. You have just created expansive, tensile and compressive stresses in the paper. Where will the paper tear? On the stress line. Stresses in steel are created by mechanical methods, such as machining, boring and forming. Residual stresses are also created in the casting or forging as a result of the differential cooling.

Thermal stresses are imparted to steel after heat treating through the quench-hardening process. An ice cube dropped into a cup of hot coffee illustrates this effect. The heating creates expansive stress on the exterior of the ice cube, while the core is still frozen. The result is stress shear or cracking due to the differing rates of thermal “growth” caused by the coefficient of expansion. Dropping a hot shotgun barrel into liquid nitrogen would have the same effect, creating stress.

Stress relief takes place when the entire mass is at an equal temperature (core and surface) and is then cycled through a wide temperature range. Steel is crystalline: think of its molecular structure like random people in a shopping mall.

In the past, we stress-relieved parts by heating them. Why is cold better? Heating a part allows only one direction of movement — expansion (the people in the mall move one step away front each other). The molecular structure starts with a random, jumbled

(stressed) organization and expands with the heat, then it returns to room temperature. The result is only a slight dislocation or stress relief (the people are still randomly located but moved one step closer).

Taking a part to extremely low temperatures creates a very dense molecular state (all the people huddle together for a zero starting location). Technically, absolute zero (-457°F) is the “zero-motion molecular state of mass.” If the rate of temperature change is slow enough, thermal compression and expansion will take place equally from the core to the surface, releasing internal stresses (the people huddle together on cooling and then, on heating, take one step apart, one at a time, resulting, at room temperature, in a uniform, marching-band lineup). The result is a homogeneously stabilized material. Understandably, this process takes a long time (more than a day) to do properly and keep the entire mass in equilibrium throughout the temperature cycling.

The deep cryogenic process also creates an increase in the wear life of steel, sometimes increasing it by over 300%. Tensile strength and an increase in toughness, as well as greater stability through the release of internal stresses, are all benefits of cryogenic processing. Deep cryogenics (below -300°F) has created many new applications in science. High-temperature superconductors, the super-conducting super-collider, cryobiology, magneto-hydrodynamic drive systems for ships and low-temperature physics all developed recently through cryogenic research.

The folks at 300 Below, Inc. have worked to refine their deep cryogenic process, known in the shooting community as “Cryo-Accurizing”. Deep-cryogenic tempering for metals has become very cost-effective. Cryogenic processing may give the greatest benefit for the lowest cost in the long run. The process is a one-time, permanent treatment that affects the entire part, not just the surface.

New and used shotguns can both benefit from the treatment. Other steel parts that receive wear, such as drill bits, surgical scissors, bearings, racing engines, slicers and knives, can also benefit from this treatment. New applications are being discovered daily. Even saxophones and trumpets have benefited, making them easier to play and giving truer notes. Some music professionals hail the process as the greatest advancement for musical instruments in the last 50 years.

The computer used in the cryogenic processing system call be programmed to duplicate the optimal cooling curve exactly within 1/10 of one degree Fahrenheit so the metal heats and cools at a slow, even rate. A contractor for NASA hailed the cryogenic system as the most accurate thermal system they had ever evaluated. The same system used for rifle barrels was used to stress-relieve an optical bench for NASA’s planetary probe.

New cryogenic machines process shotgun barrels with a controlled, dry, thermal treatment. “Controlled” simply means the process is performed according to a precisely prescribed timetable. A computer acts as the process controller to operate the descent, soaking and ascent modes. The material is cooled slowly to -300°F, is held there for many hours, then is raised to a higher temperature and slowly returned to room temperature. The “dry” process does not require the metal to be subjected to liquid nitrogen and eliminates the risks of thermal shock. The process is so gentle even a light bulb can make it through the cycle unharmed.

“Heat treating” is really a misnomer. It really should be called “cold treating.” The beneficial changes heat treating imparts to metal don’t actually take place from the heating but rather from the cooling or “quenching”. The changes don’t stop at room temperature but continue all the way, to absolute zero.

Longer Wearing Barrels
The deep cryogenic process can be thought of as an extension to heat treating. A research metallurgist at the national Bureau of Standards speaking of cryogenic processing stated, “When carbon precipitates form, the internal stress in the martensite is reduced, which minimizes the susceptibility to micro-cracking. The wide distribution of very hard, fine carbides from deep cryogenic treatment also increases wear resistance.” The study concludes: “…fine carbides and resultant tight lattice structures are precipitated from cryogenic treatment. These particles are responsible for the exceptional wear characteristics imparted to materials by the process, due to a denser structure and resulting larger surface area of contact, reducing friction, heat and wear.” The process is not a coating, but a permanent, irreversible change that goes completely through the metal!

The subjected metal develops a more uniform, refined micro-structure with greater density. “Carbide fillers” are precipitated as a result of the deep cryogenic processing. In a university study from Jassy, Romania, the carbides were shown to have tripled after cryogenic processing. The carbides fill the open spaces, or micro-voids, resulting in a much denser, coherent structure of the steel. The end result is increased wear resistance (think of glass rubbing against glass as compared to sandpaper against sandpaper). The change created is uniform throughout the steel, unlike coatings, and will last the life of the steel regardless of any subsequent finishing operations or regrinds. It is a permanent, irreversible, molecular change. Reports from precision benchrest shooters indicate barrels that normally would be “shot out” at 4,000 rounds have exceeded 10,000 rounds after deep cryogenic processing; 50-caliber barrels used for 1,000-yard competition normally considered out of specification at 900 rounds have exceeded 2,000 rounds and are still going. Heat treating is a region of science with rules not unlike cooking. Done properly, it is a dream to behold; done incorrectly, it can be a total flop. Just as a person is not necessarily a chef because they can place a cake into an oven, proper cryogenic processing requires a metallurgical education, thorough training, expert controls and the proper equipment. Be wary and check out the credentials of anyone offering such a service before you send off your gun!

Wind, temperature, vibration and personal skill are enough shooting variables to keep you busy. A moving barrel should not be part of the equation. Among the properties which define a high-quality shotgun, accuracy is of the highest importance. Deep cryogenic processing can increase the accuracy of your shotgun barrel, and it can be done in a two-day process and at a reasonable price. You just might want to look into this latest hi-tech way to improve your shooting!