Editing Rifling (section)

== Design ==
A barrel of circular [[Gauge (bore diameter)|bore]] cross-section is not capable of imparting a spin to a projectile, so a rifled barrel has a non-circular cross-section.  Typically the rifled barrel contains one or more grooves that run down its length, giving it a cross-section resembling an [[internal gear]], though it can also take the shape of a [[polygonal rifling|polygon]], usually with rounded corners. Since the barrel is not circular in cross-section, it cannot be accurately described with a single diameter. Rifled bores may be described by the ''bore diameter'' (the diameter across the ''lands'' or high points in the rifling), or by ''groove diameter'' (the diameter across the ''grooves'' or low points in the rifling). Differences in naming conventions for [[Cartridge (firearms)|cartridges]] can cause confusion; for example, the projectiles of the [[.303 British]] are actually slightly larger in diameter than the projectiles of the [[.308 Winchester]], because the ".303" refers to the bore diameter in inches (bullet is .312), while the ".308" refers to the bullet diameter in inches (7.92&nbsp;mm and 7.82&nbsp;mm, respectively).

Despite differences in form, the common goal of rifling is to deliver the projectile accurately to the target. In addition to imparting the spin to the bullet, the barrel must hold the projectile securely and concentrically as it travels down the barrel. This requires that the rifling meet a number of tasks:<ref name=lilja_accurate />
*It must be sized so that the projectile will [[swage]] or [[obturate]] upon firing to fill the bore.
*The diameter should be consistent, and must not increase towards the muzzle.
*The rifling should be consistent down the length of the bore, without changes in cross-section, such as variations in groove width or spacing.
*It should be smooth, with no scratches lying perpendicular to the bore, so it does not abrade material from the projectile.
*The chamber and crown must smoothly transition the projectile into and out of the rifling.

Rifling may not begin immediately forward of the chamber. There may be an unrifled throat ahead of the chamber so a cartridge may be chambered without pushing the bullet into the rifling. This reduces the force required to load a cartridge into the chamber, and prevents leaving a bullet stuck in the rifling when an unfired cartridge is removed from the chamber. The specified diameter of the throat may be somewhat greater than groove diameter, and may be enlarged by use if hot powder gas melts the interior barrel surface when the rifle is fired.<ref>{{cite web |url=https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/internal-ballistics |title=Internal Ballistics |publisher=[[Hornady]] |access-date=21 June 2018 |archive-date=22 June 2018 |archive-url=https://web.archive.org/web/20180622005054/https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/internal-ballistics |url-status=live }}</ref> [[Freebore]] is a groove-diameter length of [[smoothbore]] barrel without lands forward of the throat. Freebore allows the bullet to transition from [[static friction]] to sliding friction and gain linear [[momentum]] prior to encountering the resistance of increasing rotational momentum. Freebore may allow more effective use of propellants by reducing the initial pressure peak during the minimum volume phase of [[internal ballistics]] before the bullet starts moving down the barrel. Barrels with freebore length exceeding the rifled length have been known by a variety of trade names including [[Ball and shot gun|paradox]].<ref>{{cite web |url=https://www.classicshooting.com/blogs/reading/17662636-definition-of-a-paradox-gun |title=Definition of a Paradox Gun |last=Holland & Holland |author-link=Holland & Holland |publisher=Classic Shooting Company |access-date=21 June 2018 |archive-date=21 June 2018 |archive-url=https://web.archive.org/web/20180621221255/https://www.classicshooting.com/blogs/reading/17662636-definition-of-a-paradox-gun |url-status=dead }}</ref>