Killing Attacks are split into two types depending up special effects.
The default type assumes that the attack punches a comparatively small hole through an object to cause injury and death. These attacks use the Hit Location rules.
The second type is Area Effect, Explosions or other Killing Attacks defined as whole body damage.
These don't use the Hit Location rules and instead attack the total Body characteristic. Any stun multiple however does not use the 1d6-1 mechanic standard in the rules. Instead the base Stun Multiple is fixed at 3x, which of course can be modified by a Stun Modifier advantage/limit.