Polygonal rifling

Polygonal rifling (/pəˈlɪɡənəl/ 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.

Polygonal riflings with a larger number of edges have shallower corners, which provide a better gas seal in relatively large diameter bores. For instance, in the pre-Gen 5 Glock pistols, octagonal rifling is used in the large diameter .45 ACP bore, which has an 11.23 mm (0.442 in) diameter, since it resembles a circle more closely than the hexagonal rifling used in smaller diameter bores.