In various embodiments, a rotating bolt firearm may comprise an upper receiver assembly. The upper receiver assembly may comprise an upper receiver, a bolt, a barrel, and an operating system. The bolt may be installable in the upper receiver. The bolt may be moveable between a first position and a second position. The barrel may be operatively coupled to the upper receiver. The barrel may be configured to receive a forward portion of the bolt. The operating system may comprise a gas block, an adjustment, and a spring loaded detent assembly. The adjustment may comprise a threaded portion and a plurality of notches. The spring loaded detent assembly may be configured to selectively engage one of the plurality of notches in response to rotating the adjustment. The operating system may be configured to adjustably vary a gas pressure in the operating system.