What is Polygonal rifling?

Polygonal rifling 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 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 diameter, since it resembles a circle more closely than the hexagonal rifling used in smaller diameter bores.