A gas piston assembly comprises a piston apparatus having a first end and a second end. A piston bonnet has a bonnet end wall with a bonnet aperture defined therethrough and a bonnet sidewall to receive the second end of the piston apparatus therein. A piston gap is between the second end of the piston apparatus and the bonnet end wall and communicates with the bonnet aperture. An adjustable gas regulator communicates with the bonnet aperture and selectively occludes the bonnet aperture in fully open, partially open and fully closed orientations. The bonnet end wall includes a threaded bore in communication with the bonnet aperture. The gas regulator includes a threaded shaft received within the threaded bore which translates to selectively occlude the bonnet aperture. The threaded shaft includes a head portion gripped by a user while a detent engages a recess on a bottom face of the head portion.