Severing limbs in melee Options

[pster]

Would I be right to assume if I'm wielding a bladed weapon and called shot vital area (eg arm), according to SR4 I get to add a +1 to +4 to DV and an equal amount of -modifiers to hit, do I get to sever the arm off?
I'm running on the assumption if the net damage exceeds target's body then it gets chopped off. Sort of like the knockdown effect. With the exception to called shot head melee attacks, that to slice target's head off you would have to do enough damage to outright kill the target.
So if I called shot the target's arm that is wielding a weapon and managed to slice that arm off, the target loses his weapon as well.

Am I playing this right?