Cold Hammer Forging of barrels was developed during WWII to manufacture the barrels of machine guns.  After WWII, Cold Hammer Forging was further developed in Austria by Steyr. It wasn't until the 1960's that companies in the US started using it on a regular basis.  The Cold Hammer Forging of barrels is accomplished through intense hydraulic pressure applied at opposing angles by 4 carbide steel hammers. During the hammering process, a mandrel is inserted into the bore while the carbide steel hammers shape the barrel around the mandrel creating the lands and the grooves. The hammer forging process also makes it possible to create the chamber as part of the barrel, giving them the same centerline.  Additional benefits include a near defect free bore and a bore the is less likely to have fouling during the course of its lifetime.