No headgear will stop a bullet, but helmets are intended primarily to deflect eg shrapnel or falling masonry.
Today’s invention is a new form of helmet. A hardened outer shell is located, as usual, on a tight-fitting padded liner.
In this case, if the shell is subject to small, radial impacts, it will move inwards and collide with the raised brow-bumper.
In the event of a more tangential strike, the shell is free to rotate about the top boss, allowing otherwise penetrative impacts to glance off. The boss might contain a spring and damper mechanism (in both axial and radial directions) in order to dissipate a higher proportion of the ballistic energy.