Are Bulletproof Helmets really Bulletproof ?
Most modern military Bulletproof Helmets will protect against pistol caliber rounds and fragmentation/ricochets of rifle rounds. They won't protect against a direct hit from a rifle, typically. Indeed, it's very difficult to protect from rifle rounds, even if the helmet was totally bulletproof. All that energy has to go somewhere, and even if it doesn't penetrate, it will likely knock the brain around enough to kill you. And such a helmet would weigh way too much to be practical.
They generally do NOT provide face protection, but one can also get "face shields" that are basically bulletproof glass and will provide protection from the same threats that helmets do. Usually they are used in only very limited situations, as they are heavy and impractical to wear.
Ballistic 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.