July 24, 2020
Firearm historians tell us that the idea of cutting grooves in the bore of a gun barrel originated in central Europe sometime during the late 15th or early 16th century. In the beginning, it was believed that straight grooves gave blackpowder fouling a place to accumulate and that eased pushing a ball down the barrel of a muzzleloader during the loading process. Straight-groove rifling was seen more recently in sub-gauge inserts for over-under shotgun barrels made by Briley. The intent there is to improve shot pattern uniformity by arresting any spin of a plastic wad column as it travels through the bore, but it has not been widely accepted among clay target shooters. I don’t fully understand why straight-groove rifling in the barrel of an AR-15 in .223 Remington would be practical, but the Reformation rifle made by Franklin Armory has it.
Types of Rifling
It was eventually discovered that like the fletching on an arrow, spiraled or twisted rifling gave projectiles of various shapes a stabilizing spin for an improvement in accuracy. One of many unusual ways of accomplishing bullet spin early on was a spiraled hexagonal-shaped bore combined with a lead bullet of the same shape. Another form of spiraled rifling had two large grooves that accepted a belt of the same size on the bullet. Pushing a lead bullet into the barrel of either type of rifling became increasingly difficult as hard blackpowder fouling built up with each shot. A metal scraper of the same shape attached to the ramrod was used to sweep away the fouling.
Hundreds if not thousands of different forms of rifling have been developed in various countries around the world. Many originated during the age of muzzleloading firearms, and more came along during the development of breechloaders. Most of the forms of rifling we see in modern rifles today are either copies of those of yesteryear or combinations thereof. A popular example of the latter is 5R rifling that originated during the 1980s by Boots Obermeyer, who continues to turn out match-grade, cut-rifled barrels in his Bristol, Wisconsin, shop.
Obermeyer got the idea for 5R rifling while examining the bore of a Russian AK-74 rifle in 5.45x39mm. In lieu of lands with parallel sides, they are angular or sloped from top to bottom. That idea originated not with the Russians, but during the 1700s as a way of reducing the accumulation of blackpowder fouling in the bore.
Today, eliminating the sharp corner at the juncture of each land and groove has a tendency to reduce the accumulation of bullet jacket and smokeless propellant fouling. This assumes a smooth bore finish with the absence of tool marks running across the tops of the lands and grooves. Eliminating the sharp corner also makes it easier for a bullet to fully obturate, thereby preventing propellant gas from escaping around a bullet during rifling engagement. In addition to slowing erosion of the lands and grooves, copper fouling in the bore is reduced.
Rather than stopping there, Obermeyer added a fifth groove, same as in rifling adopted by the British for its .303-caliber Lee-Enfield rifles in the late 1800s and later used in the U.S. 1917 Enfield in .30-06. By positioning each land opposite a groove, bullet jacket deformation is lessened, and that reduces the possibility of jacket rupture when pushing thin-jacketed match bullets through quick-twist barrels. Beyond that, there is absolutely nothing magic about 5R rifling, and it varies considerably in both quality and accuracy among various companies that are licensed to offer it.
While many companies now offer 5R rifling, similar patterns have not been so fortunate. An example is the ratchet style of rifling that emerged during the muzzleloader era and was later used in the 1920s by cartridge developer and rifle builder Charles Newton. Ratchet rifling, as it is described today by Shilen, is offered by that company in calibers ranging from .224 to .375. Like 5R rifling, ratchet rifling has five lands and grooves.
The majority of rifle barrels made today have either four or six lands and grooves, but more and less have appeared. Among the more unusual was a barrel having an oval-shaped spiraled bore rather than more conventional lands and grooves. It appeared in some barrels made by the Ross Rifle Co. and was also tried by Newton and the U.S. military.
Back when paper-route money was sufficient to pay for several military surplus rifles each year, I owned Springfield 1903s with two-groove and four-groove barrels. The British began experimenting with two-groove rifling during the early 1800s, and beginning in 1942, Remington sought to speed up wartime production of Springfield 1903A3 rifles by fitting them with two-groove barrels. Extensive testing at Springfield Armory proved them to be as accurate as standard four-groove barrels, and acceptable accuracy life was actually extended.
I no longer have a Springfield rifle with a two-groove barrel, but my Rifles Inc. custom .300 H&H built around a Remington Model 700 action comes close with a match-grade, three-groove barrel made by Dan Lilja. That rifle was quite choosy during load development, but it eventually decided to consistently shoot three bullets inside a half-inch at 100 yards. One might think a three-groove barrel would be hard on thin-jacketed bullets, but not a single Sierra MatchKing or Berger Match Hybrid Target sent downrange has come apart prior to reaching paper targets. And considering the number of rounds fired in that rifle, I am greatly impressed each time my Lyman Borecam reveals minimal chamber throat erosion.
Increasing the number of lands and grooves in a barrel shortens accuracy life, but a considerable leap is required for it to become immediately apparent. Marlin discovered that soon after introducing the Model 322 bolt-action rifle on the Sako L46 action in .222 Remington. Whereas there had been few complaints about Micro-Groove rifling in lever-action rifles in .30-30, reports of severe accuracy deterioration at 500 rounds or so began to pour in soon after the Model 322 was introduced in 1954. The number of rounds fired was a major factor. A varmint shooter may fire more rounds in a single day than a deer hunter in a lifetime, and that made a big difference. More complaints came from prairie dog shooters in western states than from woodchuck shooters in eastern states where far fewer shots are typically fired in a day. Less than 6,000 Model 322 rifles were produced. As proved by Marlin, 12 extremely narrow lands don’t contain a lot of erosion-resisting steel. It might be compared to holding a match to thin paper and thick cardboard. Both may flame up at about the same time, but the cardboard will take longer to completely burn away.
There are claims of some types of rifling delivering higher velocities than others, but I have my doubts about it being enough to matter. Differences in bore and groove diameters and chamber dimensions have much greater influence. Match-grade, handlapped barrels will often deliver higher velocities than mass-produced barrels of the same length because their chamber and bore dimensions are held to tighter dimensions. And the difference can be quite dramatic. Of two rifles in .243 Winchester I have owned through the years, one with a 22-inch barrel made by Jarrett was consistently faster with all loads than one with a 26-inch factory barrel.
The benefit of lapping the bore of a match-grade barrel is often misunderstood. Heat-treating a barrel upsets its bore and groove diameter uniformity. Not by much and not enough to matter in mass-produced factory barrels, but it is enough to make a big difference in barrels expected to consistently deliver less than half-minute accuracy. Rifling a barrel with its bore and groove diameters a bit undersized and then handlapping to the desired diameters improves uniformity from one end of the barrel to the other. Top-tier barrelmakers guarantee a deviation no greater than 0.0001 inch from chamber to muzzle. To put that into perspective, a page in Shooting Times is 0.002 inch thick, and if you sliced it into 20 layers of equal thickness, one layer would measure the same as the end-to-end bore and groove diameter variation of a top-quality, lapped barrel. Making genuine match-grade barrels is extremely labor intensive, and while that makes them considerably more expensive, it is money well spent when the best accuracy possible is required.
Rifling twist rate, or the distance a projectile travels through a barrel while making one rotation, has changed dramatically over the centuries. A lead ball requires very little spin, and mid-1800s twist rates in military rifles were as slow as one turn in 72 inches (1:72). In other words, a ball made less than one rotation during its travel through a 30-inch barrel. The famous Rocky Mountain Rifles built during the 1800s by St. Louis brothers Jacob and Samuel Hawken had various rifling twist rates, but during about the final 30 years of production, they standardized at 1:48 twist for a .50-caliber patched ball.
Slow rifling pitches also delivered acceptable accuracy with the hollowbase Mini bullet of the American Civil War era, but they would not stabilize solid bullets of cylindrical form. This is why the trapdoor Springfield in .45-70 adopted by the U.S. Army in 1873 was introduced with a 1:22 twist in its three-groove barrel. As bullets grew longer for their caliber, rifling with even steeper pitches was required. The Krag-Jorgensen joined the U.S. military in 1892 with a 1:10 twist, and the same twist rate later worked equally well for the .30-03 and .30-06 Springfield cartridges. Some of the extremely long bullets available today require 1:7 and 1:8, but such quick pitches are not new. The barrels of the Italian Carcano of 1891 in 6.5x52mm and the Mannlicher-Schoenauer of 1903 have 1:8 twist rates.
Most rifling today has a constant rate of twist from breech to muzzle, but during the late 1800s “gain twist” enjoyed some popularity among competitors who shot custom muzzleloading and breechloading rifles at long distances. An early example began with a 1:30 twist at the breech end of the barrel and progressively increased to 1:20 twist at the muzzle. As theory had it, starting the rotation of a lead ball or bullet gently and then gradually accelerating its rotation as it traveled through the barrel subjected it to less deformation for an improvement in accuracy. Gain-twist barrels were in use as far back as the 1700s, but their biggest appearance may have been in percussion revolvers built by Colt during the 1800s. It allowed a lead ball to travel from the cylinder to full engagement with the rifling prior to being violently torqued to rotation. Gain-twist barrels are available today from several makers, but the rate gain is usually slight when compared to some of those of yesteryear. For example, a Bartlein 6mm barrel starts at 1:7.7 and gradually transitions to 1:7 at the muzzle.
Rifling in shotgun barrels has also enjoyed some use, with Paradox doubles built by the English firm Holland & Holland the most famous. The muzzle-ends of their barrels had several inches of rifling, and most were in 10 and 12 gauges. They fired hardened lead bullets loaded in cases made either from paper or thin brass. The 12-gauge blackpowder load pushed a 740-grain bullet along at 1,000 fps, and that was eventually increased to 1,200 fps by the use of Cordite propellant. Shot pattern quality from those guns was not as good as with full smoothbore barrels, but due to the slow twist of the rifling, it was not bad. Today, a number of companies offer screw-in chokes with rifling for repeating shotguns.
Shotgun barrels with full-length rifling along with saboted slug loads have transformed short-range bird guns into deer slayers capable of shooting accurately at considerable distances. Among the half-dozen slug guns in my battery, a 12-gauge Browning A-Bolt with a 22-inch, 1:28 twist barrel built in 1996 is the most accurate. At 200 yards, it consistently shoots the Federal 300-grain Trophy Copper and Remington 385-grain AccuTip slugs inside four inches.
Back to rifles, choosing the right rifling twist rate mostly depends on what it will be used for. Benchrest shooters who use the 6mm PPC for shooting groups at 100 and 200 yards stick with bullets weighing from 60 to 70 grains, and most barrels have fairly slow rifling twist rates ranging from 1:12 to 1:15. On the opposite extreme, those who bang steel and punch paper at 1,000 yards with the 6x47 Lapua and other cartridges of that caliber shoot extremely long bullets ranging in weight from 105 to 115 grains, and a twist rate of 1:7 is none too quick.
When choosing a twist rate for a big-game rifle that may be used with a wider range of bullets, I prefer to go with whatever will stabilize the heaviest (longest) bullet that will likely be used. For example, standard for the .264 Winchester Magnum has long been 1:9, but when developing the 6.5 Shooting Times Westerner, I hedged my bet toward future bullet development with a quicker 1:8 twist. Standard twist rate for the .350 Remington Magnum and .35 Whelen is 1:16 inches. When developing the .358 Shooting Times Alaskan, I went with 1:12, and it delivers excellent accuracy with bullets ranging from the Nosler 225-grain Partition through the Swift 280-grain A-Frame to the Woodleigh 310-grain Weldcore.
When in doubt about the twist rate for a new rifle, the answers are no farther away than the makers of the bullets you will shoot and, possibly even better, the maker of the barrel you will be using and the gunsmith who will be putting the rifle together.
Now that you know a thing or two about rifling, I will tell you something that some would rather you did not know. When it comes to accuracy, the form of rifling in a barrel is not the controlling factor. Match-winning accuracy comes by the use of the best steels available along with rifling formed as perfectly and with the highest level of precision possible by experienced human hands and modern machinery. Some forms of rifling are a bit easier to clean, some have a bit longer accuracy life, and some break-in a bit quicker, but when the smallest group possible is on the line, there is no “best” form of rifling. Most reputable makers of match-grade barrels who offer more than one style of rifling will agree.