For example, in movies and shows like saving private Ryan and band of brothers, sodlers are quite often shown taking a bullet to the helmet and the bullet going through with little to no trouble. It almost seems like helmets were basically pointless when it came to a direct hit. Was that accurate from back then? Whether it be due to technological/design limitations or did we just not have a good enough understanding of ballistics to make a helmet that would stop most infantry rounds? Were they meant to just protect from shrapnel or other chunks of debris but not whole bullets? Are these movies and TV shows just inaccurate? (I would understand your every day war movie, but SPR and BoB are known for they're fairly impressive historical accuracy)
I apologize if this has been asked before. I'm on mobile and can't really search effectively. Feel free to remove if need be.
There are enough leftover WWII helmets still around (as the design and materials didn't significantly change for decades) that the ones in the films are quite likely vintage. They were designed to deflect shrapnel and debris, and deflect rounds that weren't full-on, but they couldn't stop them. But remember, most of the stuff flying in wartime is not aimed directly at any one person. There are a lot more shrapnel, rocks thrown up by explosions or shooting, suppressive fire, and badly-aimed rounds than there are sniper bullets. Sometimes, even fairly direct hits could be deflected because of bullet tumble or other accidents of physics.
More importantly, even most modern Kevlar military helmets won't stop a direct round in most cases. When I was in Iraq, one of the guys in my unit was shot almost point-blank with a 9mm pistol. It entered the Kevlar of his helmet, and the layers did deflect it upward and out, so it didn't actually touch him - but it didn't just bounce off, either; the helmet was toast. If he'd been shot with, say, a 7.62 round, it would likely have punched through. How much damage it would do after that would depend on the range it was fired at; close up, it might still kill, but from further away it might just give you a bruise and get stuck between helmet and head. The new ECH will apparently stop a 7.62 round, but this was big news in 2013.
The old helmets just weren't capable; you'd have to have so much steel they'd become functionally useless. We only achieved this level of protection with Kevlar and similar materials. The tradeoff is, of course, that the steel pots were used for cooking, heating water for shaving, and other things; the new Kevlar ones have only one use.