A key operated rotary plug cylinder lock having rotary disk tumblers arranged in a pack within a rotary plug sleeve normally held against rotation within the lock casing by a locking bar which spans the shear line between the disk tumbler peripheries and the confronting wall of the rotary plug sleeve. True gates are provided in the tumblers to be aligned by an appropriate key with the locking bar to accommodate inward movement of the locking bar to a position releasing the plug for rotation within the casing, and shallower depth false gates are provided in each of the tumblers having associated false bottom recess portions in the front and rear walls of the tumblers providing forward and rearward false gate profiles or outlines simulating the profile of the true gate. Frangible tabs are also provided on each tumbler extending into a sector recess which will readily break upon excess torquing forces to prevent penetration of the lock by torquing tools.
