The Details of Accuracy
The following article was written by professional writer John Haviland and was published by Varmint Hunter magazine in the October, 2001 issue #40. It is reprinted here with the permission of the author and Varmint Hunter magazine.
The origins of accuracy or inaccuracy in a rifle barrel may be difficult to distinguish in an off-the-shelf hunting rifle that is capable of shooting mere one inch groups. Only in a properly constructed benchrest rifle will the details appear that make a barrel accurate become apparent. A couple ten thousandths of an inch variation in a barrel’s dimension doesn’t matter much in a hunting rifle, but the variation will show up in a rifle capable of shooting five shots into .125-inches or less at 100 yards.
Dan Lilja knows those details. Lilja is a barrel maker from Plains, Montana, a benchrest shooter and past director and president of the National Benchrest Shooters Association. His barrels have won every major benchrest tournament across the country. Hunters, varmint shooters, the US Armed Forces and every major bullet manufacturing company have bought his barrels for their well-known accuracy.
Knowing how and what to look for, from selecting the barrel steel to the final fitting to an action, are all necessary to make an accurate barrel.
Ninety percent of Lilja’s customers want stainless steel barrels. A good percentage of those are hunters who want the resistance to weather provided by stainless steel. Lilja uses 416 stainless. A stainless barrel has a longer and more accurate life than the standard 4140-type chrome/molybdenum steel barrel because stainless steel resists heat erosion in the throat better. “I also get a better final finish with stainless,” Lilja said.
However, there is still a demand for chrome-moly barrels. Most of the chrome-moly barrels Lilja makes are for high-grade custom hunting rifles with custom made wood stocks. And, some shooters still insist on having a blued barrel.
What steel works best depends on the makeup of the bullets intended for the rifle. For instance, for the 50 Browning machine gun cartridge a stainless steel barrel will last longer and foul less shooting conventional copper-jacketed bullets, such as the Hornady. But a barrel of chrome/moly steel will probably last longer firing lathe-turned bullets made from brass, bronze, copper or steel.
When Lilja orders the bars of barrel steel he tells the steel mill the steel is for rifle barrels. “I specify a narrower range of alloys in the steel,” he said. Lilja also expects the mill to keep the metal clean and the bars round and straight when they are rolled.
Drilling and Reaming
When the bore of a barrel is drilled, the barrel spins while the drill remains stationary. Depending on the caliber, the deep hole drill is fed into the bar at about one inch a minute. Oil is pumped through a hole in the carbide tip of the drill to flush away cutting chips and to cool the metal. If the bar is out of balance while it spins, centrifugal force will bend it. “Even though the hole is drilled in straight it won’t come out straight once its done if the barrel doesn’t spin true,” Lilja said. “The hole may only be straight while the barrel is turning. That’s why a straight and round bar is essential from the start.”
After the bore is drilled it is reamed to its final diameter. Reaming removes only a few thousands of an inch of metal, but erases the tool marks from drilling and starts the final finish in the bore.
The bore of the barrel is washed to remove the oil and chips. It is then coated with a special lubricant of Lilja’s making. With the bore ready for rifling, Lilja could choose several paths.
Hammer forging is used by many large firearms companies. The bored barrel steel is hammered around a steel rod that has a reverse image of the rifling. Machinery for hammer forging is very expensive. “About the only advantage of hammer forging is the high rate of production,” Lilja said. “Because of all the pounding, though, a hammer forged barrel has lots of stress in the metal.”
Traditional cut rifling takes many passes of a cutter down the bore to cut the grooves. Cut rifling can produce rifling as good as any method.
Lilja prefers to button rifle his barrels. “Button rifling is an efficient and easier process than cut rifling to get a quality barrel,” he said.
To button rifle a barrel a carbide plug, or button, with a reverse image of the grooves and raised lands of the rifling, is pulled down the bore by a hydraulic press to imprint the rifling. If a barrel is going to have one twist in ten inches of bore length, a guide bar with that amount of turn to it is matched on the hydraulic press to a button with the same twist.
This is a CAD solid model of a rifling button.
However, if the button is unstable while it is being pulled through the bore, a fluctuation in rate of twist can occur. “You have to pull the button through with the same amount of pressure to have control over it,” Lilja pointed out.
Lilja considers a constant rate of rifling twist throughout the bore extremely important to accuracy. If the twist decreases, a bullet traveling down the barrel is not fully supported and has a chance to yaw, or wobble, while it’s still in the barrel. Yaw is when the nose or base of a bullet spins around its center.
When Lilja started making barrels in 1985 he experimented with barrels with a twist that had a slight gain over the length of the bore. Lilja felt the gain in twist kept a more consistent grip on the bullet and kept it from wiggling in the bore. “To some extent my barrels were unique with the gain twist. And at the time I felt the slight gain in twist was beneficial to accuracy,” he said.
A .22 centerfire Lilja barrel with a 1 in 14-inch gain twist was measured for uniformity of twist by an electro-optic measuring device of Photronic Systems Engineering Company of Bonsall, California. A graph of the barrel’s twist showed six percent less than a 1 in 14-inch twist in the first inch of barrel ahead of the throat. The rifling increased to 1 in 14-inches ten inches down the bore. From there to the muzzle, 20 inches down the bore, the twist gradually increased to four percent faster than 1 in 14-inches.
However Lilja has quit using a gain twist. After years of comparing the accuracy of barrels with a gain twist and barrels with a standard twist, he has decided the gain twist offers no accuracy advantages. “You get just as good or better accuracy with a standard twist,” he said, “just as long as the twist remains exactly the same the entire length of the bore.”
A difference in groove depth as slight as two- ten thousands of an inch can cause bullet imbalance. “The geometry of the rifling has to be true all the way down the bore,” Lilja said.
The standard Lilja barrel has six lands and six grooves. The lands cover 25 per cent of the bore circumference. But land size is a tradeoff. Narrow lands distort the bullet less as it passes, but also burn away faster from powder gases. Wide lands increase pressure because of the added drag on the bullet. Lilja make some three or four groove barrels. He said the wider lands of these barrels does resist erosion and heat from powder gases better than narrower lands.
As the button is pulled down the bore and impresses the grooves and lands, metal is displaced. This causes stress in the metal, which can lead to inaccuracy, such as bullets walking off the target as the barrel heats.
Heat treating relieves that stress. Lilja worked as an industrial engineer for the John Deere Company in Iowa before he started in the barrel business in 1985. Part of his time at Deere was spent in the heat treating department. Lilja used to heat treat his barrels himself in a small oven in his shop. His increased production over the years, though, has made it easier to send out the barrels to Spokane, Washington, for heat treating.
Heat releases the stress in the metal by aligning the metal’s molecules. As the stress is relieved from the barrel, the diameter of the bore shrinks slightly. “You have to take into consideration the metal is going to snap back after rifling and the bore is going to shrink a couple ten-thousands of an inch when it’s stress relieved,” Lilja said. The measurements of drilling, reaming, rifling and stress relieving all have to be juggled to come out with a barrel of the proper dimensions. “The final groove to groove measurement should be no wider than bullet diameter,” Lilja said.
After a barrel has been taken from the furnace Lilja tests the steel for hardness. A Rockwell hardness of 24 to 25 is about right for a good accuracy life, he said.
Hand lapping is the final major step to finishing the interior of a barrel bore, and the most important for accuracy. A cast of the bore is made with molten lead and secured on a rod. An abrasive compound and oil are applied on the casting and the pushing and pulling down the bore begins. A new cast is made when the old one wears. The process is repeated, usually two or three times, until the bore is brought to its final size and finish. The amount of time spent lapping to a final finish depends on how well the barrel was reamed to remove tool marks. Each barrel takes half an hour to an hour of exertion on a rod.
While all the other processes of making a barrel are done by machine, lapping is done with a practiced hand. The man doing the lapping judges when the barrel is finished by feel and by inspecting the internal finish as it develops with a full length borescope and by measuring the diameters of the lands and grooves.
Lilja said he can feel the resistance of one ten thousandths of an inch in variation in a barrel while he is lapping. “When you get good at it you can tell when a barrel is uniform,” he said. “The art is the feel and knowing when to stop.” Lilja guarantees the uniformity of his barrels to that one ten thousandths of an inch.
Through a 75-power bore scope that projects an image of the bore on a television screen, an unlapped bore of one of Lilja’s barrels had a slightly scaly appearance. These scales are small tool marks left by reaming that are perpendicular to the bore and the bullets’ path. After lapping, though, the bore scope showed a smooth finish that ran all in the same direction as the rifling down the bore. “Having the finish run in all the same direction down the barrel causes much less bullet fouling,” Lilja said.
After Lilja or one of his workers judges the inside of a barrel is done, it is checked with the bore scope for blemishes and on a Shefield air gauge for variations in diameter. The air gauge is set for a known diameter and passed down the barrel. More air passing through the gauge reveals wide spots in the barrel. Less air shows tight spots.
The outside of a barrel can also affect accuracy. The contour of the barrel should be shaped so the barrel remains stiff. Lilja likes a fluted barrel with most barrel contours. A fluted barrel has scallops cut along most of the length of the outside of the barrel and removes a pound of steel from an average barrel. That weight can be saved or added to another part of the rifle. The larger surface area of a fluted barrel also dissipates heat faster.
The finishing step to a Lilja barrel is exterior polishing. All barrels are polished with 320 grit finish. “It’s a real messy job,” Lilja said. “That’s why we do it in a shed out back.”
Other Aspects of Accuracy
After all this a barrel is still only one part of an accurate rifle. “There’s a lot more involved in accuracy than a good barrel,” Lilja said.
The chamber of a barrel must be cut correctly and share the same center with the bore and be fitted to a quality action that is true and stiff. The action must be correctly bedded to the stock and mounted with a good scope. Bullets of known quality must also be used.
“One of the most important things is the rifle must be shot by a good shooter,” Lilja said. “Some of the problems with inaccuracy can be traced to head space problems with the shooter,” he joked.
The demand for custom barrels and rifles has continued to grow. When Lilja started making barrels in 1985 he received two hours of instructions from the man who sold him his button rifling machine. “Pretty much from there I learned everything on my own,” he said.
Since then Lilja has built a new shop on the outskirts of Plains and he and his employees have more work than they can handle. “People are more aware of custom guns probably because of articles by the good gun writers,” he said. “People know quality and if you produce a quality product they are willing to spend the money for it.”