Is it possible 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.