Is a Riot Helmet bullet-proof?

Helmets are equipment used to protect the head and are used in many places. For example, in order to prevent things from hitting the end of the hard hat, motorcycle helmet, are the defense of this function. The first function of helmet is to prevent being hit by debris, protect the head from shrapnel damage. In the battlefield, it is more terrible than bullet fragments. Wearing metal helmet can protect the head from flying shrapnel and other objects. In ancient times, soldiers going to battlefields wore gold-coded carapace and helmet. However, with the development of a gun, it is no longer feasible to defend against attack by armor. Since the beginning of World War II, grenades detonated from inside have been used in artillery shells. In order to prevent fragments from injuring the head, the function of helmet has been affirmed. When throwing grenades at each other in trench warfare, the helmet has a certain defensive effect on the grenade exploded above. At that time, the helmet was generally made of steel, not so much bullet-proof as bullet-proof, but because it had the ability to bounce off shrapnel. In practice, the penetrating force of bullets varies greatly due to the different types, distances and angles of bullets. In the experiments of helmet defense, the results are different. Bullets can easily penetrate the helmet, but at the same time, it is common that bullets do not penetrate the helmet. According to common sense, if you want to improve the defensive force of the helmet, you only need to increase the thickness of the helmet. But because the helmet needs to be worn on the head and supported by the neck, it is impossible to increase the thickness and weight indefinitely. At present, many helmet have abandoned the steel material, choose the same Kevlar as the bullet-proof vest, add this kind of fiber Gongpu after the human resin, make the shape of the helmet, bullet-proof ability has been greatly improved.