Can Bulletproof Helmets Stop a Bullet?
The short answer is yes.
Bulletproof helmets can stop a multiple bullets fired at them from various distances. The shell of the helmet is typically rated at a specific level, for example NIJ Level IIIA or III, which means it will stop bullets from that category of weapon. Depending on the design helmets can stop 9mm round, .44 Magnum bullet or even 7.62x51mm rifle round.
Detailed information on what type of bullets and fragments does the helmet stops is usually provided by manufacturer in the product specification sheet.