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In the 1960s, Paul Kopsch (an Ohio coroner), Daniel Turcos (a police sergeant), and Donald Ward (Kopsch's special investigator), began experimenting with special purpose handgun ammunition. Their objective was to develop a law enforcement round capable of improved penetration against hard targets, such as windshield glass and automobile doors. Conventional bullets, made primarily from lead, often become deformed and less effective after striking hard targets, especially when fired at handgun velocities. The inventors named their company "KTW," after their initials.
After some experimentation with sintered tungsten-alloy rounds, which were eventually abandoned due to supply and cost concerns, the inventors settled on a bullet consisting mostly of hardened brass with a steel core. In testing, the comparatively hard brass bullets wore out barrels far more quickly than standard solid lead and copper-jacketed lead rounds, since they did not deform to fit the rifling. In an attempt to reduce barrel wear, the bullets were experimentally coated with a protective layer of Teflon. The inventors, having also noted that the tips of canes were frequently covered with the relatively soft and malleable Teflon to help them grip surfaces, found that the addition of Teflon helped to prevent bullet deflection off of vehicle doors and windshields, further improving penetration against those surfaces.
By the late 1960s, KTW had begun commercial production. In 1980, continued production of the ammunition was turned over to the North American Ordnance Corporation. The production of KTW-branded ammunition eventually ceased in the 1990s. However, some manufacturers continue to coat their bullets with various compounds, notably Teflon and molybdenum disulfide, as a protective layer against barrel wear.
Not a lot of performance data is available for these bullets, although the 9mm offering was reputed to push a 100grn projectile at a velocity of 1350fps.
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In 1982, NBC ran a television special on the bullets, wherein it was argued that the bullets were a threat to police. Various gun control organizations in the U.S. labeled Teflon-coated bullets with the epithet "cop killers" because of the supposedly increased penetration the bullets offered against ballistic vests, a staple of the American police uniform. Many erroneously focused on the Teflon coating as the source of the bullets' supposedly increased penetration, rather than the hardness of the metals used and the sharp conical projectile form factor. A common resulting misconception, often perpetuated in film (i.e. Ronin (1998)) and television, is that coating otherwise normal bullets with Teflon will give them armor-piercing capabilities. In reality, as noted above, Teflon and similar coatings were used primarily as a means to protect the gun barrel from the hardened brass bullet, and, secondarily, to reduce ricochet against hard, angled surfaces. The coating itself did not add any armor-piercing abilities to bullets under normal circumstances.
Several of the various calibers of KTW rounds produced could, in fact, penetrate police vests, under certain conditions. However, as Kopsch pointed out in a 1990 interview, "adding a Teflon coating to the round added 20% penetration power on metal and glass. Critics kept complaining about Teflon's ability to penetrate body armor... In fact, Teflon cut down on the round's ability to cut through the nylon or Kevlar of body armor." 
The federal ban on armor-piercing pistol ammunition uses only the composition of the bullet's core to determine legality. However, many individual states have legislation restricting various kinds of coating materials. For example:
- Alabama state law provides that "the possession or sale of brass or steel teflon-coated handgun ammunition is illegal anywhere within the State of Alabama".
- North Carolina state law specifically forbids persons in that state to "import, manufacture, possess, store, transport, sell, offer to sell, purchase, offer to purchase, deliver or give to another, or acquire any Teflon-coated bullet".
- Oklahoma - Teflon-coated bullets are illegal in Oklahoma under some circumstances.
- Oregon state law forbids the possession of any handgun ammunition, the bullet or projectile of which is coated with Teflon while committing or intending to commit a felony.
- Pennsylvania state law provides that "It is unlawful for any person to possess, use or attempt to use a KTW teflon-coated bullet or other armor-piercing ammunition while committing or attempting to commit" certain enumerated "crime[s] of violence".
- South Carolina state law specifically bans "ammunition or shells that are coated with polytetrafluoroethylene (Teflon)".
- Virginia state law specifically bans "bullets, projectiles or other types of ammunition that are: coated with or contain, in whole or in part, polytetrafluoroethylene (Teflon) or a similar product" while committing or attempting to commit a crime.
- Armor-piercing shot and shell
- Black Talon, a type of expanding, hollow-point ammunition also vilified in the early 1990s as "cop-killer bullets".
- "History Of Federal Ammunition Law". Institute for Legislative Action. National Rifle Association. 29 July 1999. Retrieved 25 November 2014.
- "Cartridge of the Month". Cartridgecollectors.org. Retrieved 2012-08-20.
- AL Code § 13A-11-60
- NCGS § 14-34.3
- Okla. Stat. Ann. tit. 21, § 1289.19
- ORS 166.350(a)
- "Title 18 - PA General Assembly". Legis.state.pa.us. Retrieved 2014-03-11.
- SC Code 16-23-520
- VA Code 18.2-308.3