Editing Rifling (section)

== Variable pitch rifling ==
A ''gain-twist'' or ''progressive rifling'' begins with a slow [[#Twist rate|twist rate]] that gradually increases down the bore, resulting in very little initial change in the projectile's [[angular momentum]] during the first few inches of bullet travel after it enters the [[gun barrel#Bore|throat]].<ref>{{cite web | url=https://www.forgottenweapons.com/ask-ian-progressive-twist-gain-twist-in-small-arms/ | title=Ask Ian: Progressive Twist (Gain Twist) in Small Arms? | date=21 July 2022 }}</ref> This enables the bullet to remain essentially undisturbed and trued to the case mouth.  After engaging the rifling at the throat, the bullet is progressively subjected to [[accelerate]]d [[angular momentum]] as it is propelled down the barrel.  The theoretical advantage is that by gradually increasing the spin rate, torque is imparted along a much longer bore length, allowing thermomechanical [[stress (mechanics)|stress]] to be spread over a larger area rather than being focused predominantly at the throat, which typically [[wear]]s out much faster than other parts of the barrel. Gain-twist rifling was used prior to and during the [[American Civil War]] (1861–65). Colt [[Colt Army Model 1860|Army]] and [[Colt M1861 Navy|Navy]] revolvers both employed gain-twist rifling. Gain-twist rifling, however, is more difficult to produce than uniform rifling, and therefore is more expensive. The military has used gain-twist rifling in a variety of weapons such as the {{val|20|u=mm}} [[M61 Vulcan]] Gatling gun used in some current fighter jets and the larger {{val|30|u=mm}} [[GAU-8 Avenger]] Gatling gun used in the A10 Thunderbolt II close air support jet. In these applications it allows lighter construction of the barrels by decreasing chamber pressures through the use of low initial twist rates but ensuring the projectiles have sufficient stability once they leave the barrel.  It is seldom used in commercially available products, though notably on the [[Smith & Wesson Model 460]] (X-treme Velocity Revolver).<ref>{{cite web|url=http://www.smith-wesson.com/webapp/wcs/stores/servlet/Product4_750001_750051_765500_-1_757771_757767_757751_ProductDisplayErrorView_Y|title=Product: Model 460XVR™|access-date=2014-12-31|archive-date=2016-09-15|archive-url=https://web.archive.org/web/20160915175038/http://www.smith-wesson.com/webapp/wcs/stores/servlet/Product4_750001_750051_765500_-1_757771_757767_757751_ProductDisplayErrorView_Y|url-status=live}}</ref>