Inside a Rifled Barrel
Rifling is the set of spiral cuts machined into the inside of a barrel, and it converts the projectile's forward motion into spin. That spin acts like a gyroscope, keeping the projectile stable in flight. Reading a barrel specification correctly means understanding the two surfaces rifling creates and the diameters measured across each.
Lands and Grooves
The grooves are the recessed spiral channels cut into the bore, and the lands are the raised original bore surfaces left standing between them. As the projectile passes, the lands press into its bearing surface and engrave it, gripping the metal so forward motion becomes rotation. This engraving is what leaves the characteristic marks on a fired projectile.
Bore and Groove Diameter
Bore diameter is measured across the tops of opposing lands, giving the smaller of the two figures, while groove diameter is measured across the bottoms of opposing grooves, giving the larger. Both appear in dimensional standards, and the difference between them equals twice the groove depth. A projectile is typically sized to the groove diameter so it fully fills the rifling and seals propellant gas.
Twist Rate
Twist rate states how far the projectile travels to complete one full rotation of the rifling, written as a ratio such as 1:7 (one turn in seven inches). A smaller second number means a faster twist and more spin. Faster twists are chosen to stabilize longer, heavier projectiles that need more rotational speed to stay point-forward.
Groove Count and Direction
Rifling has a defined number of grooves, commonly four, five, or six, and a twist direction that is either right-hand or left-hand. These attributes describe the rifling pattern rather than its performance, and they are one way forensic examiners distinguish barrels. The count and direction are fixed features of how a barrel is cut or formed.
Cut, Button, and Polygonal Rifling
Rifling can be produced by cutting each groove, by pulling or pushing a hardened button through the bore to press the grooves in, or by hammer-forging around a mandrel. Polygonal rifling replaces sharp lands and grooves with rounded, hill-and-valley contours. Each method yields the same spin-imparting result through different geometry.
Worked Example
Consider a .308 barrel with a bore diameter of 0.300 in and a groove diameter of 0.308 in. The groove depth is half the difference, or 0.004 in, and a 0.308 in projectile fills the grooves while the 0.300 in lands engrave it. With a 1:10 twist, that projectile completes one rotation every ten inches of travel.
A Common Misconception
People often assume the projectile's stated diameter should match the bore diameter measured across the lands. In fact the projectile is sized to the larger groove diameter, so that it seals against the grooves while the lands cut into it. Confusing the two figures makes the projectile seem oversized when it is correctly matched.