A gas piston assembly (406) comprises a piston apparatus having a first end and a second end. A piston bonnet (508) has a bonnet end wall (534) with a bonnet aperture (533) defined therethrough and a bonnet sidewall (514) to receive the second end of the piston apparatus therein. A piston gap (532) is between the second end of the piston apparatus and the bonnet end wall and communicates with the bonnet aperture. An adjustable gas regulator (562) 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 (568) in communication with the bonnet aperture. The gas regulator includes a threaded shaft (564) received within the threaded bore which translates to selectively occlude the bonnet aperture. The threaded shaft includes a head portion (574) gripped by a user while a detent (584) engages a recess (582) on a bottom face of the head portion.