Action disabling device in a firearm having among its conventional components a spring-closed breech bolt, a tubular magazine, and a pivoted cartridge lifter. The disabling device is in the form of an operating arm pivoted with one end to a fixed part of the action so as to be swingable with its other end into active and inactive positions in which the same is in and out, respectively, of the closing path of the breech bolt, and is normally spring-urged into its inactive position, with this operating arm being, on overtravel of the lifter in lift direction for lack of a cartridge thereon, engaged and taken along by the lifter into its active position to stop the breech bolt on partial closure of the same. The operating arm is one of the arms of a double-arm lever the other arm of which projects to the outside of the receiver and is there accessible to an operator's finger for swinging the operating arm to its inactive position.