Bulletproofing is the process of making something capable of stopping a bullet or similar high velocity projectiles e.g. shrapnel. The term bullet resistance is often preferred because few, if any, practical materials provide complete protection against all types of bullets, or multiple hits in the same location.
In 1887, Dr. George E. Goodfellow of Tombstone, Arizona Territory, documented three cases where bullets had failed to penetrate silk articles of clothing. He described the shooting death of Charlie Storms by gambler Luke Short. Although shot in the heart, "not a drop of blood" exited Charlie Storms' wound. Goodfellow found though the bullet did indeed kill Charlie Storms, it failed to pass through a silk handkerchief, essentially catching the bullet, but it was not enough to stop the bullet entirely. Another was the killing of Billy Grounds by Assistant City Marshal Billy Breakenridge. Goodfellow examined Billy and found that two buckshot grains had penetrated Billy's thick Mexican felt hat band embroidered with silver wire, penetrating his head and flattened against the posterior wall of the skull. Another of the grains had passed through two heavy wool shirts and a blanket-lined canvas coat and vest before coming to rest deep in his chest. But Goodfellow was fascinated to find in the folds of a Chinese silk neckerchief around Grounds' neck two shotgun pellets but no holes and no wounds. And he described a wound to Curly Bill Brocius who had been shot through the right side of the neck, narrowly missing his carotid artery. A portion of his silk neckerchief was carried into the wound by the bullet, preventing a more serious injury, but the scarf was undamaged. The Tombstone Epitaph reported, "A silken armor may be the next invention."
Goodfellow wrote an article for the Southern California Practitioner titled "Notes on the Impenetrability of Silk to Bullets". He experimented with designs for bullet-resistant clothing made of multiple layers of silk. By 1900, gangsters were wearing $800 silk vests to protect themselves.
Bullet-resistant body armor has been in use since about 1984. When law enforcement began wearing body armor, there was a dramatic drop in officer deaths, saving over 3,000 lives as a result. The National Institute of Justice first developed standards for ballistic resistant body armor in the 1970s. The standards have been revised five times since 1984. The National Law Enforcement and Corrections Technology Center tests body armor to assess its compliance with the standards and publishes the results.
Bullet designs vary widely, not only according to the particular firearm used (e.g. a 9×19mm Parabellum caliber hollowpoint handgun cartridge will have inferior penetration power compared to a 7.62×39mm rifle cartridge), but also within individual cartridge designs. As a result, whilst so-called "bullet-proof" panels may successfully prevent penetration by standard 7.62×39mm bullets containing lead cores, the same panels may easily be defeated by 7.62×39mm armor-piercing bullets containing hardened steel penetrators.
Bullet-resistant materials (also called ballistic materials or, equivalently, anti-ballistic materials) are usually rigid, but may be supple. They may be complex, such as Kevlar, Lexan, and carbon fiber composite materials, or they may be basic and simple, such as steel or titanium. Bullet resistant materials are often used in law enforcement and military applications, to protect personnel from death or serious injuries.
There are various mandatory tests which items must pass before they can be classified as bullet-resistant. These tests specify the detailed characteristics of bullets which the material or object must be resistant to. For example, the United States National Institute of Justice standard 0104.04 for bullet-resistant vests specifies that a Type II vest must not deform clay representing the wearer's body when hit by an 8.0 g (124 gr) 9 mm caliber round nosed full-metal jacket bullet travelling at up to 358 m/s (1175 ft/s); but a Type IIIA vest is needed for protection against the same bullet travelling at up to 427 m/s (1400 ft/s). In both cases, the vest is not required to protect against a second hit within 51 mm (2 inches) of the first.