BARRELS AND THE RIFLING WITHIN THEM:
PUTTING A NEW TWIST ON THE STORY
Sources: NRA Firearms Source Book,
NRA Basic Rifle, Shotgun, Pistol, Muzzleloader.
Greg Knox, Owner of HGR, Wikipedia.
THE development of the barrel has been a 600 years on going process. It’s a careful blend of chemistry, physics, metallurgy, engineering, and ballistics. It’s primary function is to contain hot, rapidly expanding propellants (gases, lead, shot swarm, ) of chemical energy converted into kinetic energy. Of all the components of the firearm, the barrel is the most important component in determining accuracy. Even in a shotgun, the barrel design controls the tightness and quality of the pattern.
Despite differences in form, the common goal of rifling is to deliver the projectile accurately to the target.
The barrel has a long cylindrical hole running through it. It maybe sealed on one end, or not depending on the firearm. This hole is called the “BORE”. The beginning design was just a smooth interior. The propellant was “Black Powder” or “Gun Powder”. Most people don’t know this history but black powder only burned about 50% of the powder load. That means 50% of the powder remained inside the barrel. Each time it was fired, it layered the barrel with it’s residue, and after a few shots, the bullet would no longer fit down the barrel. It had to be cleaned frequently, and that was not always possible in the heat of a battle.
RIFLING was invented near the end of the 15th century. The problem: how to extend the immediate use of the rifle without having to clean it. Solution: dig Grooves into the barrel to allow the residue to drop out of the way. So that was the original purpose of rifling. To keep the bore cleaner longer. As a side benefit, accuracy improved. Further development added a twist to give the bullet a rotational stability and enhanced accuracy ( In 1520 August Kotter, an armourer of Nuremberg, Germany improved upon this work.). Rifling appeared in military firearms about 100 years later. The TWIST is either left hand or right hand depending on the designer. The TWIST RATE is the amount of spin or pitch the bullet rotates along the distance of the barrel, such as 1 turn in 10 inches or 1 turn in 20 inches (which are modern standard rates). Muzzleloaders rate of turn is 1:24 to 1:48 inches per turn, much slower rates. The optimum twist rate will provide best accuracy, and adjusting the rate changes the accuracy and bullet stability. Testing is done at common muzzle velocities and ambient temperatures.
In addition to imparting the spin to the bullet, the barrel must hold the projectile securely and concentrically as it travels down the barrel. This requires that the rifling meet a number of tasks:
- It must be sized so that the projectile will swage or obturate upon firing to fill the bore.
- The diameter should be consistent, and must not increase towards the muzzle.
- The rifling should be consistent down the length of the bore, without changes in cross-section, such as variations in groove width or spacing.
- It should be smooth, with no scratches lying perpendicular to the bore, so it does not abrade material from the projectile.
- The chamber and crown must smoothly transition the projectile into and out of the rifling.
When the projectile is swaged into the rifling, it takes on a mirror image of the rifling, as the lands push into the projectile in a process called engraving. Engraving takes on not only the major features of the bore, such as the lands and grooves, but also minor features, like scratches and tool marks. The relationship between the bore characteristics and the engraving on the projectile are often used in forensic ballistics.
TWIST TYPES: Constant – Rate Twist
: the most common, the twist rate remains the same from one end of the bore to the other end. Gain – Rate Twist
: a slow beginning at the chamber end and increasing rotation as it travels towards the muzzle. This is more commonly found in muzzleloaders than modern firearms. They are harder to make which increases the cost of manufacturing.
LANDS and GROOVES
are the components of the rifling. The LANDS are the high point of the carving, and the GROOVES are the embedded or dug out bottoms of the rifling. A barrel of circular cross-section is not capable of imparting a spin to a projectile, so a rifled barrel has a non-circular cross-section. Typically the rifled barrel contains one or more grooves that run down its length, giving it a cross-section resembling an internal gear, though it can also take the shape of a polygon, usually with rounded corners. Since the barrel is not circular in cross-section, it cannot be accurately described with a single diameter. Rifled bores may be described by the bore diameter (the diameter across the lands or high points in the rifling, CALIBER (BORE SIZE) is the diameter of the distance between the Lands (the 2 highest points measured in millimeters or inches). or by the GROVE DIAMETER (the 2 lowest points measured in millimeters or inches).
NOTE: Differences in naming conventions for cartridges can cause confusion; for example, the projectiles of the .303 British are actually slightly larger in diameter than the projectiles of the .308 Winchester, because the ".303" refers to the bore diameter in inches, while the ".308" refers to the groove diameter in inches (7.70 mm and 7.82 mm, respectively). Literature and research is necessary when considering
Conventional Lands & Grooves are sharp corners on the shoulders of the corners. Other types include Polygon, Ratchet, Hybrid, and Enfield. Normally it’s an even number and lands are opposite of lands. When there’s an odd number, and the groove is opposite a land. This is call an “ENFIELD” or “5R” rifling arrangement.
Barrel materials consist of a variety of alloys of steel and are preferred because they are widely available, inexpensive, easily machined, strong, tough, hard (25-32 on the Rockwell C scale), heat and wear resistant, and shock stable. Early barrels were made of brass or iron. In 1865 smokeless powder was introduced and not suitable for brass or iron.
Most are Alloy Steels (popular is US SAE 4140 chrome-molybdenum, aka Moly Chrome steel). The alloys offer strength vs. cost balance.
Stainless Steel barrels have been around since 1930’s. They have gained rapid popularity because they are resistant to heat erosion and resistant to rust and corrosion. A popular stainless steel barrel is made of SAE 416R containing 10% Chromium and small amounts of sulfur. These barrels are more expensive to make.
Titanium – cobalt alloys are stronger and half the weight. However, it’s hard to machine and very expensive. Barrels made of titanium-cobalt are required to have a steel sleeve inside them. Some barrels are carbon fiber and have a titanium sleeve.
Fiberglass barrels have a steel liner.
Chrome plating may be used in barrels to extend the life by protecting it from heat (machine guns or ammo with corrosive primers) but it is difficult to get a consistent thickness with chrome. Incidentally, aircraft engine cylinders can be lined with chrome for heat protection. Other coatings such as Gas-nitriding, steel liners, molybdenum disulphide, cryogenic treatments (competition shooting barrels: soaked for hours in extremely cold solutions to improve accuracy and barrel life), are subject to further writings by HGR.
MANUFACTURING STYLES of barrels is usually done in 1 of 7 methods. These methods are derived from the old methods which were done by hand one grove at a time. In the 1700’s rifling machines were in wide use. Shotgun barrels were made by twisting alternating ribbons of steel and iron together around a mandrel and hammering them together to form the barrel bore. This method is known as “DAMASCUS” barrels and worked for black powder, but they are to weak for smokeless powder.
Most rifling is created by either:
- cutting one groove at a time with a machine tool (cut rifling or single point cut rifling);
- cutting all grooves in one pass with a special progressive broaching bit (broached rifling);
- pressing all grooves at once with a tool called a "button" that is pushed or pulled down the barrel (button rifling);
- forging the barrel over a mandrel containing a reverse image of the rifling, and often the chamber as well (hammer forging);
- flow forming the barrel preform over a mandrel containing a reverse image of the rifling (rifling by flow forming)
Most manufactures (except hammer forged designers) use this general method. The barrel composition is specified by the gun manufacturer and made to order by the steel mill. They normally come in 12 ft sections or longer. These are normally stressed relieved (heated to 600 F and then slow air cooled). The INITIAL TRUEING stage the barrels are cut to length 28-30 inches and turned on a lathe to make it round. DEEP HOLE DRILLING is done using a long drill with a hollow stemmed tungsten-carbide tip. This tip only has 1 cutting tooth. High pressure oil is forced through the hollow stem to provide cooling and to flush away the chips away. Some spin the barrel, some spin the bit, and some spin both in opposite directions. Drilling in done with 5/10,000th of an inch accuracy. REAMING is next. This makes the interior surface smooth with no scratches. If there are defects they are normally called “Chatter marks”. RIFLING is done now, usually by CUTTING, BROAD, or BUTTON methods. These are the modern methods:
CUT RIFLING - a cutter resembling a fish hook is pulled through the bore shaving a thin layer each time with multiple cuts to the needed depth. This is the oldest method. It maintains close tolerances and twist rate can be adjusted. Widely use for test barrels. It is a very time consuming process. The barrel bore must be lapped afterwards to make it smooth.
BUTTON RIFLING – an extremely hard tungsten carbide bullet-shaped cutting button is push or pulled through the bore which displaces the steel in it’s way. It’s fast and done in one pass of the cutter without needing to be lapped. This bore is very accurate. Do to the high stress, the barrel must be stress relieved again. This process is expensive and to change the twist rate requires re-tooling.
BROACH RIFLING – similar to the Cut Rifling method, the broach method has multiple fish hooks and can cut the whole barrel with one pass. It’s fast for mass production with little stress on the bore, but the broach itself is expensive. The barrel must be lapped when done.
HAMMER-FORGED RIFLING- very common for mass production. Similar to a chisel, the steel can be hammered cold or hot. Hot is faster but more expensive. Cold process leaves a high quality barrel. Since no metal is removed, the barrel is harder and more sturdy (metal pounded and compressed together). Machines are expensive and not readily adjustable. Higher quality but NOT MATCH GRADE. Outer surface has marks that have to be removed.
CATION RIFLING – the process of removing metal through acid. Produces a very tight tolerance barrel. No cutters, buttons, or mandrels are used. Easily adjustable rate. Very expensive, HAZMAT storage required, and the process is still being perfected.
RIFLING PATTERNS: a more in depth look at the types of Rifling patterns reveals these types:
CONVENTIONAL: the corners are cut sharp and flat on the lands / grooves. There may be a rounding of the land top. It’s an inexpensive proven technology that works well. Other forms may be better suited for special bullets. The sharp edges cut into the bullet, and gases leak at the bottom of the grooves and fouling collects in those cavities.
POLYGONAL RIFLING: is a series of curves forming the polygon (no sharp edges). They appear to be smooth bore viewed from the muzzle. Gives good gas sealing for higher velocity, easier to clean (less shavings), longer barrel life. Will not stabilize a non-jacketed bullet. Best suited for hammer forged barrels.
The grooves most commonly used in modern rifling have fairly sharp edges. More recently, polygonal rifling, a throwback to the earliest types of rifling, has become popular, especially in handguns. Polygonal barrels tend to have longer service lives because the reduction of the sharp edges of the land (The grooves are the spaces that are cut out, and the resulting ridges are called lands) reduces erosion of the barrel. Supporters of polygonal rifling also claim higher velocities and greater accuracy. Polygonal rifling is currently seen on pistols from CZ, Heckler & Koch, Glock, Tanfoglio, and Kahr Arms, as well as the Desert Eagle.
HEXAGONAL RIFLING: Made famous in the mid 19th century by the British (Whitworth design). The 6 sided rifling was form fitted for the bullet. Lands / Groves on the barrel matched the bullet reducing fouling, improving ballistics, faster loading, and longer barrel wear. The disadvantage was only that type of bullet worked, and it was an expensive process to make both.
Extended range, full bore concept
For tanks and artillery pieces, the extended range, full bore concept developed by Gerald Bull for the GC-45 howitzer reverses the normal rifling idea by using a shell with small fins that ride in the grooves, as opposed to using a slightly oversized projectile which is forced into the grooves. Such guns have achieved significant increases in muzzle velocity and range. Examples include the South AfricanG5 and the GermanPzH 2000.  Gain-twist rifling Gain-twist rifling begins with very little change in the projectile's angular momentum during the first few inches of bullet travel after ignition during the transition from chamber to throat. This enables the bullet to remain essentially undisturbed and trued to the case mouth. After engaging the rifling the bullet is progressively subjected to accelerated angular momentum as burning powder propels it down the barrel. By only gradually increasing the spin rate, torque is spread along a much longer section of barrel, rather than only at the throat where rifling is eroded through repeated rifling engagement.
ASPECTS OF BARREL QUALITY can determine the accuracy of the shot groups. Steel: Inclusions, localized variations in hardness or composition, imperfections, and the manner it is drilled, reamed, rifled, and lapped are all factors in the accuracy of groups. As the barrel ages, the groups size and position may drift.
Dimensional Uniformity: The width and depth of the lands / groves, bore diameter, and twist rate should be uniform. Top quality variance is less than .0003/10,000th of an inch. A device called “air gauge” is often used to determine the uniformity of the bore.
Chamber alignment: misalignment may occur in 2 ways: Skewed (the axis of the chamber is angled to the axis of the bore. Or, the chamber axis may be parallel to the bore but offset to it. Either way causes the bullet to enter the bore at an angle causing the bullet to “bore yaw”, or to lay in the bottom if cut to deep. Alignment is critical.
Throat Configuration: Throat diameter, length, and angle all determine how the bullet travels from the case mouth into the rifling. The optimum bullet placement is with the throat length that puts the bullet lightly in contact with the rifling or just short of contact.
Muzzle Crown: as a bullet exits the barrel the hot expanding gasses exit after the bullet, if it’s seated properly. A muzzle crown that is not square to the axis of the bore will allow the gas to escape on one side faster than the other which causes the bullet to yaw. Your accuracy will suffer. A nick caused by a cleaning rod cleaned from the muzzle end can cause this. Brass rods or Stainless Steel are better than aluminum rods.
Bore Finish: a smooth bore will not pick up as much lead / copper from the bullet as it moves down the bore. J&B’s bore cleaner and J&B’s bore polish work wonders to keep the bore smooth and clean. There is a balance though, too smooth also leads to more fouling. Use the cleaners as directed for maximum efficiency
Bore Taper: Competition barrels bore taper is none. Production models have larger tapers, and some taper from the muzzle back to the chamber which squeezes the bullet. So it may be necessary to ream the chamber at the end of the barrel with the larger bore diameter. Thread and Shoulder Alignment: most rifle barrels are threaded into the receiver until stopped by the shoulder of the barrel where it butts against the receiver face. Precise alignment with the axis of the bore and shoulder must be machined square to the bore axis to get the best accuracy.
BARREL ENHANCEMENT has several options available to improve accuracy.
Cryogenic Barrel Treatment: Mentioned earlier, this process involves slowly cooling the barrel to -300 F temps for several hours. Afterwards it is slowly returned to room temperature. This is suppose to remove metal stress accumulated in the manufacture process, however tests show it actually only removes 6% of the stress accumulated. The stainless barrels of 416R composition show no benefits of the treatment. The treatment is suppose to change the metal molecule composition grain, smoother bore, reduces fouling, and reduced wear. NRA testing showed 10% treatment in some barrels, and zero in the rest. Barrel life seemed to be extended somewhat claims higher velocity, reduced metal fouling (thus easier cleaning), and increased barrel life. There are 2 methods of applying the Moly. One is by shooting molly coated bullets through the barrel until the coating transfer is complete. The other is by soaking the barrel in Molybdenum where the particles 3/10 micron in size attach to the barrel in an efficient method. US Military uses this method. This second method is endorsed by many competitors. Fire Lapping: This process fires bullets with abrasive materials through a barrel to lap the bore. It may be purchased in kit form offering the shooter a variety of abrasive materials. A course material is started with older pitted barrels. This process shows accuracy improvement in older barrels, but is not recommended by most manufacturers for new barrels. New ones are normally lapped at the factory. Aggressive lapping will change the bore caliber. Caution is recommended. Barrel Fluting is the process of removing exterior barrel material along the longitudinal axis (along the length of the barrel). This may reduce the weight on a short barrel, or maintain the weight by lengthening the barrel. Additional cooling is also attained by exposing more surface area to exterior air. Six and eight flutes are common. Misconceptions include: 1) fluting requires additional stress relief. False, properly fluted barrels will maintain it’s current stress relief. 2) Fluting adversely effects accuracy. Tests show there is no loss of accuracy.
Barrel Tuning Barrels develop harmonic vibrations when they are made. As they wear, or especially as they are cleaned from the muzzle end and scratched or nicked, the harmonic balance is disturbed. Remarkable performance may be restored by cutting as little as 50/1000th of an inch from the barrel, making sure it is squared to the axis of the bore. Another method is by using a Browning device called the Ballistic Optimizing Stabilization System (B.O.S.S.). This device, installed by the factory, is threaded into the muzzle. It’s a combination muzzle break and tuning weight. It’s adjustable so you can find the best accuracy as the tuning changes with the different loads. These devices are the same concept of harmonic tuning devices used for law enforcement radars or those in musical instruments.
Hummer Barrels is a termed by shooters
BARREL MAINTENANCE Barrel Break-In Every barrel, regardless of how smooth it’s been machined, has defects from drilling, reaming, rifling, etc. As a bullet passes through the bore it removes these defects but often leaves pieces of copper embedded in the bore metal. Those copper pieces collect fouling.
Breaking in a barrel is recommended in the following processes.
Clean the barrel between each shot for 15-25 shots.
Clean the barrel between each group of shots for 10 groups
Clean the barrel between several 5-10 shot strings. The key to knowing when breaking in is complete, a noticeable decline in fouling debris
This process applied to rifled barrels, not smooth bore such as shotguns.
Barrel Cleaning This topic is commonly covered in all NRA courses.
Bore Cleaners Traditional liquids (Outers, Hoppes’ etc.) are used. Some are specialized, such as the JB bore cleaner and JB bore polisher, some use copper mesh to clean, some are chemical foams, some use electricity at low levels to strip the fouling, and finally there is Ultrasonic systems which use sound waves through liquids to remove fouling.
Fouling Shots are sometimes used after extensive cleaning to leave a fresh layer of copper fouling in the bore (the extensive cleaning removed all copper residue that was in the bore). After 15 to 50 shots, accuracy begins to deteriorate. Thanks for your interest. I hope you found this article interesting, informative, and educational.
Comments and suggestions on future topics are appreciated.
Your article "Barrels and the Rifling Within Them" is just perfect. Started shooting at age four with my father. I started punching paper with .22's and went from there. I am still amazed at what there is to learn.
I have long wondered how those little spirals down the inside of the barrel got there. Your article gives a very well written and very readable account of the basics. I now understand what I want to know as the layman shooter. You have found the right balance of detail and brevity to allow me to know how the process works and how it affects the final product.
Thank you for a great article. I will continue to learn, but your brief has taught me more usable information than any other I have read of its kind.
Glenn A. Tatum
The Intro picture of the Glock barrel was
a "TEACHABLE MOMENT" in an instructor
class. Three Glocks of different calibers
were disassembled and cleaned by the
instructor applicants. The 40 caliber
barrel was assembled into the 45 caliber
frame, thus the gap in the slide / barrel
<<<< to the left ... assorted training guns.