An adjustable firearm gas block has a gas channel intersected by a threaded bore. A set screw with longitudinal slots is driven through the bore until it enters and constricts the gas channel. A second bore, parallel to the first bore, is connected to the first bore by a passage. A spring and a detent plunger having a slanted notch are inserted in the second bore with a bearing located in the slanted notch and passage. When the detent plunger is in a resting position, it presses the bearing into a longitudinal slot, preventing the set screw from turning. When the detent plunger is urged away from the resting position, the bearing disengages the longitudinal slot, traveling into the slanted notch and allowing the set screw to rotate, thereby adjusting the gas pressure emitted by the gas block.