What is a polygonal pistol barrel?

Most rifled firearms have a bore that is round, and has spiral grooves (the rifling) cut inside the bore. However, some guns (notably most Glock pistols) use a different system. The cross section of the bore is a polygon, with several flat surfaces. These surfaces twist in a spiral as you pass up the bore. It is claimed that produces a higher velocity than standard rifling, but it cannot be used with unjacketed lead bullets.