Rate of Twist
Definition
The barrel length required for the rifling to complete one full helical revolution, expressed as a ratio (e.g. 1:9 means one complete rotation per nine inches of barrel).
- Definition
- Barrel length per full rifling revolution
- Notation
- Ratio, e.g. 1:9 inches
- Function
- Stabilises the bullet by imparting spin
- Forensic use
- Narrows the class of barrels that could have fired a bullet
Common questions
How does an examiner use rate of twist to narrow down a suspect firearm?+
Measuring the angle of the land and groove impressions on a fired bullet against its diameter lets the examiner calculate the twist rate that produced them, which is then compared against reference data for known barrel manufacturers and models to narrow the class of possible firearms.
Why does rate of twist need to match the bullet weight used?+
A twist rate too slow for a heavy, long bullet fails to stabilise it in flight, causing tumbling and poor accuracy, while too fast a twist for a very light bullet can over-stabilise or even damage it, so manufacturers pair barrel twist to their intended ammunition range.
Related terms
- BKA
- Bundeskriminalamt, the German Federal Criminal Police Office, which operates the German GRC and ballistic identification database feeding the European EBIS network.
- CFSL
- Central Forensic Science Laboratory: a central government lab operating under DFSS. India has CFSLs at New Delhi, Hyderabad, Chandigarh, Kolkata, Pune, Guwahati,...
- General Rifling Characteristics (GRC)
- The class-level barrel properties (land count, groove count, land width, groove width, twist direction, twist rate) that allow a fired bullet to...
- Grooves
- The recessed channels cut into a rifled barrel between the lands. They create raised land impressions on a fired bullet's surface.
- Lands
- The raised ridges of a rifled barrel. Lands engrave groove impressions into the surface of a passing bullet. They are the harder...
- Left-Hand Twist (LHT)
- Rifling that spirals counterclockwise toward the muzzle. The defining house convention of Colt Manufacturing Company and of the Lee-Enfield rifle family.
- NABIS
- National Ballistics Intelligence Service; the UK police intelligence service that maintains a national headstamp and firearm reference collection and coordinates ballistic intelligence...
- Polygonal Rifling
- A rifling form that uses a rounded hexagonal or octagonal bore profile instead of discrete lands and grooves. Characteristic of Glock barrels;...
- Right-Hand Twist (RHT)
- Rifling that spirals clockwise toward the muzzle when viewed from the breech. Used by the majority of the world's firearm manufacturers.