Bulletproof Ball Cap? GOOD: Stops .40 cal BAD: Does not stop traumatic concussions

Of course, the impact from a round is still going to do some damage, and comes well within the range of force that can cause a serious concussion. When compared to the damage caused by an unimpeded bullet, however, it could prove to be a worthwhile investment.
The hat starts with a base model in three different sizes. From there, the team modifies it to include a BulletSafe ballistic pad in the front two panels of the six-panel hat. The team expects that it can stop frontal impacts from handguns that use 9-mm, .40 and .38 caliber rounds.

Well, its a start of an idea.  Notice it is a Kickstarter proposal.

via BulletSafe Bulletproof Hat offers protection with a low-key design.