A real ballistic helmet isn't "bulletproof" either. Not rifle caliber anyway. The best ones can stop +P 9mm and .44mag, but are heavy as fuck. "Ballistic" in this sense means fragments.
Kids in basic would argue whether or not the helmet would stop a bullet. The argument ended the day a canteloupe was put in a k-pot and blew the fuck up at 300m with an M4 using M193.
Since none really stop bullets and the bullets that they can stop are usually anemic, losing weight and gaining vision and hearing and head movement (especially tilted back in prone) always meant more to me. IMO, that helmet Crye makes is very hard to beat, just over 2lbs. and ballistic rated (2 different ratings and weights in fact). Plus they're ventilated.
Some guys tested out several helmets in an Amazon video I was watching. What was interesting was that they pretty much all stopped x round (within NIJ standards for such) but each deformed differently and some deformed to the point of being lethal. I seriously doubt you could tell the difference in the helmets if they were stripped and painted uniformly either, so buyer beware I guess.
And yeah, pretty much any rifle cartridge will punch a hole through one.