a gas cylinder to be connected in a fluid-tight manner to a hole in a firearm barrel via a gas channel; and

a control element movable between first and second positions, wherein in the first position the control element opens the gas channel by forming a fluid-tight connection, and wherein in the second position the control element closes the gas channel by interrupting the fluid-tight connection, wherein the control element includes a bore, a first sealing element, and a second sealing element, the bore arranged between the first sealing element and the second sealing element along a longitudinal axis of the control element, wherein the gas block includes a recess to accept the control element, the recess includes a first stopping surface to maintain the position of the control element in the first position and a second stopping surface to maintain the position of the control element in the second position.