Despite the limitations, more than 22 million M1 helmets were made during the war. Despite all of the advances, even today's high-tech, lightweight military-grade ballistic helmets aren't bulletproof.

But even the M1 "steel pot" helmet made from manganese issued in 1942 for WWII (and remained in service until the mid-1980s) wasn't meant to be bulletproof. Its primary goal was to deflect ...