Head shot

Coup de grace.

Head shot certainly doesn't work. A head shot is a professional 8 x 10 glossy photograph of an actor or model, sent to prospective employers as part of his or her portfolio.

What about "cranial shot".

Nicholas is correct.

A head shot is an instantly fatal shot to the head, usually fired by a sniper. It's also a common term among players of first-person-shooter video games.

And, in the spirit of Hurley, I offer you: noggin pop.

Apparently "dead checking" is the modern terminology used by US solders. http://en.wikipedia.org/wiki/Dead_checking

Coup de grâce (blow of mercy) might also be of use.
http://en.wikipedia.org/wiki/Coup_de_gr%C3%A2ce

Is "jargon" a uncountable noun? I was taught that "jargon" is and was corrected in this usage by my English teacher. Since this site is where English usages are scrutinized, I am just puzzled and raise my question. I am not being picky...I really want to know the answer from native speakers.

Thank you all!

Monkey, like most mass nouns, it depends on how you're using it. Usually, it is uncountable and used in the singular. While we're at it, so is "usage". You should have said: "...this site is where English usage is scrutinized..."

Thank you so much. It is really difficult for non native speakers to sense such subtle difference as you pointed out. Thanks again! :)

Coup de grâce is certainly the one term I am familiar with as being associated with a firing squad