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Conventional eight groove rifling on the left, and octagonal polygonal rifling on the right. Polygonal rifling (/ p ə ˈ l ɪ ɡ ə n əl / pə-LIG-ə-nəl) is a type of gun barrel rifling where the traditional sharp-edged "lands and grooves" are replaced by less pronounced "hills and valleys", so the barrel bore has a polygonal (usually hexagonal or octagonal) cross-sectional profile.
Rifling of a 105 mm Royal Ordnance L7 tank gun Conventional rifling of a 90 mm M75 cannon (production year 1891, Austria-Hungary) Rifling in a GAU-8 autocannon. Rifling is the term for helical grooves machined into the internal surface of a firearms's barrel for imparting a spin to a projectile to improve its aerodynamic stability and accuracy.
The means of measuring a rifled bore varies, and may refer to the diameter of the lands or the grooves of the rifling. [2] For example, the 257 Roberts and 250 Savage both use a .257 inch projectile; both 250 Savage and 257 Roberts rifle bores have a .250 inch land diameter and .257 inch groove diameter. [4]
In 5-R rifling, the "side" of the land is cut at a 65° angle, rather than 90° in conventional rifling. This results in less barrel fouling, and more consistent point of impact, compared to conventional rifling when relatively high numbers of rounds are fired between cleaning, as might be expected in military applications.
SAAMI recommends a barrel with a 6 groove contour and a twist rate of one revolution every 9 in (229 mm) with each groove having a width of .095 in (2.41 mm). The recommended bore diameter is .256 in (6.50 mm) and the groove diameter is .264 in (6.71 mm). SAAMI recommends that rifling commence at .221 in (5.61 mm) from the chamber mouth.
The distance across the bore from groove to groove is greater than the distance from land to land. Projectiles fired from rifled barrels must be of the full groove-to-groove diameter to be effectively rotated by the rifling, but the caliber has sometimes been specified as the land-to-land diameter before rifling grooves were cut.
In declining to step aside from two high-profile Supreme Court cases, Justice Samuel Alito on Wednesday provided a rare window on the opaque process by which justices decide to step aside from cases.
Another issue that has an effect on the barrel's hold on the bullet is the rifling. When the bullet is fired, it is forced into the rifling, which cuts or "engraves" the surface of the bullet. If the rifling is a constant twist, then the rifling rides in the grooves engraved in the bullet, and everything is secure and sealed.