Are Military Helmets Bulletproof ?
The Kevlar helmets the troops use isn’t designed to be bulletproof. Such a helmet would be excessively heavy and cause neck fatigue quickly. Engineers keep trying though.
The helmets are designed to reduce injuries from flying debris, ricochets and stray bullets. They will stop a bullet provided you are far enough away from the shooter and bullet velocity/energy is low enough. But ammunition type plays a big role here too.
The current U.S. military helmet, designated the ACH (Advanced Combat Helmet) replaces the PAGST helmet used up until about 2003. The ACH is designed to defeat thecommon 124 grain 9mm handgun bullet fired from certain distances with a 90% confidence of no penetration. This is fire aimed and directed at the helmet.
The helmet is not designed to stop rifle rounds which typically have velocities up to three (3)times that of a 9mm bullet and carry much higher energy levels. The ACH can (and has)stopped indirect hits from rifle bullets that have ricocheted off an armored vehicle or the side of a building, but this is quite different than stopping a bullet aimed at the head/helmet.
A head shot in combat is not common at normal ranges. Most of the time, a shot to the head is the result of a sniper. Proper sniper rifles fire rounds that are significantly more powerful than the standard 5.56 or 7.62. No helmet is going withstand a hit from 308 Lapua.
However…grenades, rockets, and mortars are encountered often in modern combat. Part of these weapons is an effective fragmentation range for shrapnel. That's the real reason why you wear your vest and helmet. When any of the above weapons explode a short distance from you, you want all the protection you can get. Coincidentally, when you have to haul ass to cover to avoid getting blown to bits, you don't think too much about a oft landing. Once again, helmet saves the day by allowing you to keep your brains where they belong and not leaking on the ground somewhere in Buttfuckistan. Also important when the vehicle you are riding in encounters an IED and your head may end up banging against the inside of the vehicle.
But more than occasionally, American troops come under fire from some fucker who sets an ambush and tries his hand at sniping with a 5.56 or 7.62. And if you happened to be wearing the older PASGT helmet designed for up to 9mm type rounds, you're dead if you're the target du jour. So the ECH design was made. Designed to stop or deflect up to .30 caliber rounds (that would include the 7.62x51 or .308 as we call it in the US). It's also lighter and is designed to have better ergonomics. That's important when it's 110°F out and you have to wear a damn helmet.