The barrel is the rifle’s delivery system, the steel guidance mechanism that sends the projectile spinning toward the target. Barrel technology has come leaps and bounds in the last century, to the point where the accuracy has become both highly predictable, as well as repeatable. It’s important to know how barrels work in order to better understand how a bullet will perform within its confines.
THE THROAT
Starting at the breech end, your barrel has three or four main parts, depending on the type of firearm. For rifles, as well as semi-automatic pistols, there is a chamber, throat or leade, and the rifling itself, all terminating at the crown. The chamber is a mirror image of the cartridge to be fired and is sealed by the breech bolt or block to ensure all the burning gas pushes things toward the muzzle end of the barrel. The throat, or leade, is the area between the chamber of the barrel and the point where the rifling begins. The length of the throat can vary greatly, from less than 1⁄16 inch, to as much as ½ inch, depending on the cartridge and manufacturer. The throat is exposed to burning powder and hot gas, and when shooting a high-velocity cartridge is often the first part of the firearm to show wear and erosion. Some of the fastest cartridges, like the .300 Remington Ultra Magnum and .264 Winchester Magnum, can show throat wear in as little as 1,500 rounds. I make a conscious effort not to heat my barrels excessively, to help keep wear and tear to a minimum. Some companies (Weatherby for example) purposely extend the throat of their barrels to give room for the bullet to jump. This is known as free-bore, and can help increase accuracy. You never want a modern cartridge to have the projectile touching the rifling; dangerous pressures can easily develop. At the end of the throat, the rifling begins.
RIFLING
Rifling is the set of twisted ridges you’ll see when you look down the bore of the firearm. It imparts a spin on the bullet, keeping it stable in flight. Those ridges, properly called lands, engrave their imprint into your bullet, and are machined at a smaller diameter than the bullet itself. The corresponding valleys, or grooves, are designed to be at caliber dimension to properly seal the gas and build pressure. The number of lands and grooves can vary, from the two-groove U.S. Army Springfield rifles of the early 20th century, to the Marlin MicroGroove barrel that used 16 or more, and all sorts in between. (Note: some handgun companies today employ polygonal rifling, which is a bit of a different geometry, yet works fine for their purposes.) Almost all common barrels use a static twist rate, meaning that the grooves are cut in a specific manner to maintain a consistent spin on the bullet. When researching rifles, note the barrel
