Ballistics 101: What Is Rifling?

105mm_tank_gun_Rifling

Previously in our introductory series on ballistics, we’ve discussed the concept of caliber, as well as ballistic coefficient and its close relative form factor. Today, we’re going to look at the concept of rifling, and how it relates to bullet stability.

First, though, we’ll need to define a term:

  • Bore: The hole drilled in the barrel of a firearm, through which the bullet travels when it is fired.

The earliest firearms had simple tubular bores, like a modern shotgun. In the late 1400s, gunsmiths in what is now Germany developed a method of scraping grooves in barrels to allow room for blackpowder and lead fouling to build up, hopefully prolonging the accuracy of the barrels before they needed to be cleaned. They found, eventually, that by twisting these grooves in a barrel, they could dramatically improve its accuracy. These grooves became known together as “rifling” and they are a key technology in modern rifle (as the name suggests) and pistol barrels.

rifling

A cross-section of 8-groove rifling. The red highlighted segments are “grooves” and the distance between opposite grooves is called the “groove diameter”. The blue highlighted segments are called “lands”, and the diameter between opposite lands is called “bore diameter”.

 

Rifling works by spinning the projectile about its axis, causing gyroscopic forces that spin-stabilize it throughout its flight; tighter rifling will spin a bullet faster, while looser rifling will spin a bullet slower. The rate at which the bullet is spun is very closely tied to how stable it will be; too slow, and the bullet will tumble in the air, too fast and the bullet may break apart in flight. The speed of rotation caused by rifling is determined by the rifling’s pitch or twist rate, which describes how loose or tight the rifling is twisted inside the barrel. Usually, rifling twist in the United States is described by a phrase like one turn in N inches, often shortened to “1:N” or “1/N”, which refers to the distance over which the bullet would complete one full revolution.

For example, the US Army’s M4 Carbine has a twist rate of one turn in 7 inches (“1:7” or “1/7”), which is quite tight. An M4 Carbine with a fairly new barrel firing 5.56mm M855 ammunition will spin the projectile at over 5,000 rotations per second to stabilize its flight!



Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


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  • PK

    “Tighter” and “looser” isn’t the right way to describe the twist rate, and may lead to confusion about land/groove diameter controlling spin along with the twist rate.

    • ostiariusalpha

      I wonder if Carlucci or Chinn has any reference to twist rates as “tight” or “loose.” That might clarify how informal or not the use of those terms really is.

      • Tim

        Most industries that use twist in relation to the product, eg. cable, textile, etc., refer to a longer or shorter pitch (or twist rate or strand). Long and short terminology tie right into the definition of pitch which would be the distance for one full revolution. Technically, twist rate is the inverse of pitch, i.e. a pitch of 5 inches per revolution, or 1:5, is a twist rate of 0.20 revolutions per inch.

  • therealgreenplease

    At some point you guys should do a piece on gain-twist rifling 🙂

    • Tim

      Also, no mention of polygonal rifling? Where are all the Glock folks…

  • Budogunner

    Dat bore tho…

  • Tim

    My question is if there are any standards or customs regarding the direction of rifling? Is everything right-hand twist or no??
    I first wondered about this when reading about fluted self defense bullets, which appear to be directional.

  • Louis Bethel

    Great article and refresher. Many of us that work in shops take this type of info for granted.

  • Todd

    Is this correct? “The blue highlighted segments are called “lands”, and the diameter between opposite lands is called “bore diameter”. If so then how does a .308 bullet work in a .308 bore? I always assumed the bore was the groove dia, considering how else could it work, and the obvious grooves cut into the bullet.