If you’ve ever observed the barrel of a typical handgun or rifle, you may have noticed a series of grooves inside. These grooves form the rifling of that barrel, which is essential to most guns’ performance, accuracy, and effective range.
Understanding how rifling works and how rifled barrels contribute to a firearm’s performance is essential education for any responsible gun owner.
The History of Rifling
Before rifles became commonplace, the primary infantry firearm was the musket. Although it was deadlier and easier to train with than a crossbow, the musket was not an accurate weapon, especially at long range. Muskets have what armorers call a smoothbore barrel, a simple metal tube with no grooves or cuts inside.
As guns loaded with blackpowder and bare lead projectiles, muskets accumulated significant fouling with each shot fired, requiring frequent cleaning to prevent jams.
German armorers in the late-15th century initially cut straight grooves down the barrel, giving space for the soot and grime to deposit into, in the hopes of increasing the number of shots between each cleaning.
These armorers eventually discovered that by twisting these grooves, the projectiles fired were much more accurate and reached much longer ranges, resulting in the birth of the first rifle.
What is Rifling and How Does It Work?
All gun barrels possess a bore, which is the hollow cavity through which the bullet travels. The bore is slightly smaller than the projectile’s diameter (within a few thousandths of an inch of tolerance), allowing the bullet to remain snug and straight inside the barrel.
A conventional rifling machine cuts channels into the bore, forming two alternating segments: the lands (which are the un-cut sections of the barrel) and the grooves (which are slightly wider than the bore).
The phenomenon called spin-stabilization occurs when firing a bullet through a rifled barrel. The projectile engages the grooves, causing it to rotate as it travels through the tube. This rotation imparts a certain amount of longitudinal spin on the projectile as it leaves the barrel and begins traveling downrange. As the projectile spins around its longitudinal axis, it remains stable in flight.
When comparing projectiles fired from rifled barrels vs. smoothbore barrels, the spin-stabilization imparted to bullets fired from the former allows them to fly over a longer distance and with greater accuracy than a non-spinning projectile.
These differences are the reason why muskets were abandoned for rifles for long-distance engagement.
Twist Rate and Projectile Behavior
Although the general principle that bullets launched from rifled barrels are usually more stable and accurate than those fired from smoothbore barrels, it is critical to understand that not all barrel rifling is the same.
The rate at which a specific projectile will spin in the air depends on the barrel’s twist rate. Twist rate notation is written “1 in N” or “1:X,” where X is a measurement of length (inches in the US and the UK, millimeters everywhere else).
The twist rate corresponds to the distance a bullet must travel inside the barrel to complete one full rotation. For example, a twist rate of 1:7 means the bullet will complete one full revolution every 7 inches.
A higher number means the barrel’s twist rate is slower (or looser), whereas a lower number means the twist rate is faster (or tighter).
However, each projectile requires a specific twist rate to achieve sufficient stabilization without ill effects. The general rule suggests that the longer and heavier the bullet, the faster the ideal twist rate.
Too fast, and it risks undergoing the effects of over-stabilization (the projectile risks breaking apart or behaving unexpectedly upon hitting the target). Too slow, and the bullet will not achieve sufficient stabilization, eventually yawing or tumbling, significantly reducing accuracy and terminal effectiveness.
Types of Rifling
If you’ve ever shopped for gun parts, various terms on in-store signage may have caused you to ask yourself, “What is button rifling?” or “Why is a polygonal barrel important?”
Virtually all gun barrels made today possess either traditional rifling or polygonal rifling.
During the manufacturing process of a conventional gun barrel, the gun barrel starts out as steel rods, which must first be reamed to form the bore, then cut using specialized machinery to create the rifling. The exact process used determines whether the barrel will feature traditional or polygonal grooves.
Although modern-manufactured barrels are typically very accurate, regardless of the rifling style, it is critical to understand the differences between traditional and polygonal rifling and how each affects bullet behavior inside and outside the barrel.
Traditional Rifling
Traditional rifling processes cut standard grooves with square edges into a cylindrical bore, following the same basic principles first discovered in the 15th century. If you own a Beretta 92FS or a 1911 pistol, they most likely possess traditionally rifled barrels.
Traditional Rifling on a Ruger LCP
There are multiple ways to manufacture barrels with traditional rifling. The three most common methods used today are cut rifling, button rifling, and broach rifling.
Pros of traditional rifling:
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Generally more accurate, better suited for rifles, especially rifle platforms where accuracy at extended ranges is critical.
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Traditional grooves have more depth to accommodate fouling and metal deposits, making traditional rifled barrels safer when regularly shooting unjacketed lead projectiles.
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Easier and less expensive to manufacture, making traditional barrels more readily available.
Cons of traditional rifling:
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Traditional lands and grooves bite into the bullet more aggressively, causing more bullet deformation and increased drag when traveling through the barrel and resulting in slightly lower muzzle velocity when exiting.
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The deeper grooves accumulate more fouling, requiring more cleaning and maintenance.
Polygonal Rifling
Polygonal rifling, such as that seen in a Glock or Heckler & Koch pistol, usually hammer or press the bore into the desired polygonal shape using techniques such as the cold hammer forging method.
Polygonal Rifling on a Glock 19
Pros of polygonal rifling:
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The less pronounced grooves mean a polygonal rifled barrel has more metal than an equivalent traditional barrel, resulting in higher durability and service life.
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The rounded corners of polygonal lands and groves cause less bullet deformation, causing less drag and higher muzzle velocities.
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Polygonal rifling creates a more efficient gas seal, resulting in fewer propellant gases escaping past the projectile, increasing muzzle velocity consistency.
Cons of polygonal rifling:
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The manufacturing processes are more expensive and out of reach for all but the largest and most advanced gun manufacturers.
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Polygonal barrels are generally considered to be less accurate than traditionally rifled barrels. For this reason, they are rarely encountered outside of handguns (although some high-end sniper rifles do use polygonal rifling, such as the renowned PSG-1).
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Shooting non-hard cast, unjacketed lead bullets through a polygonal barrel causes rapid lead fouling, requiring more frequent cleaning and increasing the chances of a barrel failure.
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