A security screening apparatus for differentiating between ferromagnetic objects having different magnetisation comprises at least one magnetic sensor (106) arranged to produce a signal indicative of an ambient magnetic field or gradient over a zone of sensitivity (128) around the sensor, and a signal processing circuit which receives as an input the signal from the magnetic sensor and a detection circuit that determines whether a signal is above a detection threshold, which, in response to a detection, emits an alert signal; and detection zone defining means for defining a detection zone within the zone of sensitivity, the detection zone including an inner bound at a predetermined distance from the magnetic sensor. The detection zone defining means is configured such that objects with a large magnetisation, such as rifles, shotguns or shrapnel suicide vests, are detectable within the detection zone to the extent that they cause an alert signal to be emitted, whilst objects with a smaller magnetisation do not cause an alert to be emitted. The zone defining means could be a physical barrier (130) such as a pole or fence.