Most current bulletproof helmets meet the national standards of level 2 to 4, which correspond to the American NIJ IIIA level
Bulletproof helmets can indeed stop bullets, but they come in different protection levels. Different levels can withstand different types of bullets.
Most current bulletproof helmets meet the national standards of level 2 to 4, which correspond to the American NIJ IIIA level. The V50 value is usually above 600m/s (meaning it can withstand fragments or bullets traveling at 600m/s). A qualified helmet within its protection level can likely withstand frontal attacks by bullets. They can generally resist shotgun pellets as well as most handgun bullets, including providing protection against certain angles of rifle bullets. The round shape of the helmet, at the right angle, has a high probability of deflecting bullets coming from the side, and attachments on the outside of the helmet sometimes offer additional protection.
There is a chance, albeit small, that a rifle bullet could be stopped frontally. The battlefield is complex, and there are always times when one is against a wall. Helmets can withstand the kinetic energy of handgun bullets, and rifle bullets that pass through walls, rocks, boards, trees, or are deflected by obstacles, lose energy during flight, which weakens their kinetic energy, allowing them to be stopped.
"Bulletproof" doesn't mean bullets can't penetrate; it means that with each layer of protective material a bullet passes through, it loses some kinetic energy until it is stopped by the bullet-resistant fiber material.