Draksila's right. Zombies don't suffer the multipliers of any type (bullet, location, slash/stab, etc), including hit location. If the zombie has Weak Spot: Brain and someone shoots it in the head with a .22 handgun (for example), it'll only do the D4x2 damage and nothing more.

However, you should note the information under Effects of Damage (p.147). Those are used even if a zombie has a Weak Spot. So, if someone takes aim at the kneecap of a zombie that has Weak Spot: Brain with his trusty 12-gauge and hits with 20 points of damage, the leg becomes so much hanging gristle and bone and the zombie most likely falls over. Not out, but certainly not going anywhere fast. Keep in mind that the attacker must do at least the amount it says there for each body part in one shot to make the limb useless.
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How many times is someone gonna feed me sheeps lungs cooked in its own stomach before they get that I dont like it!
-Ron Stoppable-
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