[Lazamataz]

The so called cop killer bullets were hard with a Teflon coating that would help penetrate.

Negative. The teflon coat was meant to prevent barrel damage. The round was hardened, and would rip up the barrel. The teflon did no penetrating and aided in no way.

Common mistake.

[mountainlion]

The teflon coat was meant to prevent barrel damage

Also reduced friction, heat, barrel fouling and helped increase velocity.