Ask yourself why the rest of the ENTIRE world uses some form of the 7.62, yet we use the inferior 5.56 You do realize that Russians are amused we like 7.62x39mm AK's so much, since they use 5.45x39mm now? ...which is basically 5.56mm. I'm not a huge fan of 5.56, but it's far from the entire world using 7.62.    As for bulletproof helmets, there's one little problem here: generally, a helmet that can stop a bullet isn't going to stop the kinetic energy, just the penetration. On a lot of rounds, the kinetic energy is enough to snap your neck. So for many cases, it doesn't matter whether or not there's a hole in your head, because the alternative is dying from a broken neck.    Of course, high collars on body armor does serve to reduce the amount of neck snap you can get, but at a cost of visibility.