[alexey-salmin]

Older steel helmets don't but modern IIIA-class helmets (e.g. ECH) do.

Kinetic energy is not an unsolvable problem, energy can be dissipated. The momentum is in fact the problem that can't be really worked-around (except spreading it over longer period of time) but the momentum of a bullet is low.

UPD replaced "impulse" with "momentum", lost in translation

[sobriquet9]

Level IIIA resists penetration from .357 SIG, 9 mm, and .44 magnum. Those are handgun rounds. It may stop some intermediate rounds if you’re lucky, but certainly not full power rifle rounds.

Helmets cannot deform as much as vests before seriously injuring the wearer, limiting their capacity to dissipate kinetic energy. And if you make them too rigid, concussion becomes a problem.