Abstract: An apparatus for sensing the force of a collision and actuating a vehicle occupant restraint system in response thereto. An actuator piston is slidably mounted within a restraint system module and is disposed intermediate a source of pressurized gas and an inflatable occupant restraint. The actuator piston is maintained in position by a frangible sensor shaft intermediate the source of pressurized gas and the inflatable occupant restraint system. A sensor weight is annularly disposed about the frangible sensor shaft. The sensor weight comprises a non-uniform mass which, when subject to the inertial force of a collision, will transmit a shearing force to the surface of the sensor shaft which is sufficient to fracture or otherwise disrupt the structural integrity of the sensor shaft.