[A_D_E_P_T]

The bullet's kinetic energy is absorbed by the deformation and fragmentation of the projectile, the deformation of the helmet shell, the deformation/compression of rigid helmet pads, and conversion to heat.

The momentum of a projectile impact is very low, far lower than head contacts in sports. See, e.g.: https://i.ibb.co/7X6YCLD/ballistic-head-impact-updated.jpg -- from https://nap.nationalacademies.org/read/13390/chapter/9

If your helmet stops a bullet, there's some risk of head injury if the helmet shell, upon deformation, comes into contact with your skull. Otherwise you'll be okay. There's some historical information on this at: https://www.ade.pt/bulletproof-helmets

Also, the kinetic energy of a fragment near the site of an explosion can be higher than the kinetic energy load of a 9mm handgun bullet, which is generally the only small-arms threat that combat helmets are rated to stop.