The M3 is an automatic, air-cooled blowback-operated weapon that fires from an open bolt. Constructed of plain 0.060-inch-thick (1.5 mm) sheet steel, the M3 receiver was stamped in two halves that were then welded together.[11] The M3 has a fixed firing pin milled into the face of the bolt and fires using the principle of advanced primer ignition blowback operation. The bolt was drilled longitudinally to support two parallel guide rods, upon which were mounted twin return (recoil) springs. This configuration allows for larger machining tolerances while providing operating clearance in the event of dust, sand or mud ingress.[24] The M3 features a spring-loaded extractor which is housed inside the bolt head, while the ejector is located in the trigger group.[25] Like the British Sten, time and expense was saved by cold-swaging the M3's barrel.[11]