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. Who didn't teach Sherlock gun safety?
Can we appreciate that Sherlock HAD bags of crisps? He doesn't eat, I doubt he regularly carries chips for John. He KNOWS when Molly goes for lunch, anticipated it. Doesn't care for the solar system, knows when Molly eats.