Editing Rifling (section)

== Projectiles ==
[[File:7,62x39 bullets - unfired and fired.JPG|thumb|upright|left|57-N-231 standard [[7.62×39mm]] military bullets with steel core - the one on the left is unfired, the one on the right is fired, with the rifling grooves visible. Notice the copper wash scraped off and the steel jacket is exposed on the groove marks]]
[[File:7,62mm G3 oder MG3.jpg|thumb|upright|Three recovered [[7.62×51mm NATO]] bullets (next to an unfired cartridge), showing rifling marks imparting anti-clockwise spin]]

The original firearms were [[muzzleloader|loaded from the muzzle]] by forcing a ball from the muzzle to the chamber. Whether using a rifled or smooth bore, a good fit was needed to seal the bore and provide the best possible accuracy from the gun. To ease the force required to load the projectile, these early guns used an undersized ball, and a patch made of cloth, paper, or leather to fill the ''windage'' (the gap between the ball and the walls of the bore). The patch acted as a [[wadding]] and provided some degree of [[seal (mechanical)|pressure seal]]ing, kept the ball seated on the charge of [[black powder]], and kept the ball concentric to the bore. In rifled barrels, the patch also provided a means to transfer the spin from the rifling to the bullet, as the patch is engaged rather than the ball. Until the advent of the hollow-based [[Minié ball]], which expands and obturates upon firing to seal the bore and engage the rifling, the patch provided the best means of getting the projectile to engage the rifling.<ref>{{cite book |title=The Complete Blackpowder Handbook: The Latest Guns and Gear |author=Sam Fadala |publisher=Gun Digest |year=2006 |isbn=0-89689-390-1}} Chapter 18, The Cloth Patch</ref>

In [[Breech-loading weapon|breech-loading firearms]], the task of seating the projectile into the rifling is handled by the ''throat'' of the [[Chamber (firearms)|chamber]]. Next is the ''freebore'', which is the portion of the throat down which the projectile travels before the rifling starts. The last section of the throat is the ''throat angle'', where the throat transitions into the rifled barrel.

The throat is usually sized slightly larger than the projectile, so the loaded cartridge can be inserted and removed easily, but the throat should be as close as practical to the groove diameter of the barrel. Upon firing, the projectile expands under the pressure from the chamber, and obturates to fit the throat. The bullet then travels down the throat and engages the rifling, where it is engraved, and begins to spin. Engraving the projectile requires a significant amount of force, and in some firearms there is a significant amount of freebore, which helps keep chamber pressures low by allowing the propellant gases to expand before being required to engrave the projectile. Minimizing freebore improves accuracy by decreasing the chance that a projectile will distort before entering the rifling.<ref>{{cite book |title=Handbook for Shooters & Reloaders Volume II |author=P. O. Ackley |publisher=Plaza Publishing |year=1966}} pages 97–98</ref><ref>{{cite web |url=http://www.riflebarrels.com/articles/50calibre/throats_50_bmg.htm |title=Thoughts on Throats for the 50 BMG |author=Daniel Lilja |access-date=2008-08-19 |archive-url=https://web.archive.org/web/20080513011603/http://www.riflebarrels.com/articles/50calibre/throats_50_bmg.htm |archive-date=2008-05-13 |url-status=dead }}</ref>

When the projectile is swaged into the rifling, it takes on a mirror image of the rifling, as the lands push into the projectile in a process called ''engraving''. Engraving takes on not only the major features of the bore, such as the lands and grooves, but also minor features, like scratches and tool marks. The relationship between the bore characteristics and the engraving on the projectile are often used in [[forensic ballistics]].