WOW. Read this article NOW. It's a simply ingenious interpretation of Sherlock's mind palace scene in His Last Vow. It's things like this that make me say that Sherlock is my favorite show of all time, and my favorite fandom of all time.
The best thing about this is that John is completely comfortable with Sherlock holding a gun to his head. John, the ex-soldier who has already been shot once, is completely comfortable with this situation.