How to create a bulletproof helmet ?
There’s a big difference between a helmet that will stop a .22 LR at 300 yards range and one that will stop 7.62x51 point blank.
A sheet of 3/8″ AR500 armor steel will stop .50 BMG ball ammo at 300 yards. You could make a helmet out of 3/8″ steel, but it would be too heavy to wear.
The current Advanced Combat Helmet gives NIJ IIIA level protection and weighs about 3 - 3.5 lbs; that’s about a pound lighter than the old Kevlar PGAST helmet, which also had a IIIA threat rating.
Level IIIA is rated to stop 6 hits from 124 grain 9mm round travelling at 1400 fps.
Level IV is rated to stop one hit of .30–06 armor piercing.
Bullet-resistant is about the best you can do.
In addition, it could’t be just a helmet. It would have to be a part of a larger piece of armor that could prevent serious, even fatal, neck injuries.
To prevent penetration by a high-powered projectile, the helmet is going to have to defect/absorb a lot of energy over a relatively small area compared to body armor. The weak link in the chain is the neck. Injuries to the spine would be similar to those seen in judicial hangings that employ a measured drop to break the neck.
Such a piece of armor would seriously limit the wearer’s vision and mobility.
Basically, if you want a bulletproof helmet, get a tank.