Dump a half magazine of 9mm on up on some guy wearing armor and he will definitely be an unhappy camper.

Helmets don't protect a whole lot head on. The main thing is overhead and impact protection and reduction of injury from explosions. Delta Force back in the 1990s was using skateboard helmets. Their idea was it was lighter and cooler than a PASGT, gave a better mounting base for NV hear, and gave a little more head protection than a boonie hat. They also felt that it would offer some level of IFF.

Try checking your targets carefully the next time you go target shooting and block off the area that is covered by a helmet, and you will figure out that most of the target still looks pretty much the same. A headshot at handgun distances is a headshot and 90% chance of being a fight stopper, with round 75-80% chance of being lethal.