Firearms with adjustable gas block regulators have a receiver defining a passage receiving a reciprocating bolt assembly, a barrel defining a barrel axis and extending from the receiver and defining a barrel gas aperture, a gas block connected to the barrel and having a gas block inlet registered with the barrel gas aperture, the gas block defining a gas block bore transverse to the barrel axis and in communication with the gas block inlet, the gas block bore defining a gas block bore axis, the gas block defining a gas outlet in communication with the gas block bore, a regulator closely received in the bore and operable to reciprocate along the gas block bore between at least a first position and a different second position, the regulator defining first and second gas paths having different gas flow characteristics.