Fair enough. It is perhaps worth noting that people who are shot in the head can and do sometimes survive. A quick Google of "man shot in head survives" (without the quotation marks) threw up a number of links to news articles, of greater or lesser repute.
As regards the Gunslinger's called shot ability... it would probably suit you better if they called it something other than "headshot". In that regard, it's actually like a lot of my issues with 4e - taken as
mechanics it works fine, but I had problems reconciling it because of some of the
terminology used. Perhaps it should be called "head graze" instead?