Yeah, don't rely blindly on his books; a lot of it is accurate, but bits are entirely fictional. The thing about stabbing the medulla through the occipital bone is absolutely correct, though.
I got an email from Clancy once, actually, following a conversation about the MP5/10, which of course ends up in his books as the MP-10. Turns out that the FBI have their own internal designation (much like the US military calls the Minimi the M249, while the Swedes call it Ksp90), calling it the MP-10, so it's not entirely wrong.