Got a job for you, Marked One...
5.7x28mm AP is an unused ammo type which was partially included in S.T.A.L.K.E.R.: Shadow of Chernobyl.
A relatively new cartridge made for the FN P90 and the FN Five-seven pistol by FN Herstal. The 5.7x28mm is a middle ground between pistol cartridge, such as the 9x19mm used by the MP5 and intermediate assault rifle rounds, such as the 5.45x39mm, used by the Ak-74 and its variants. Though never displacing pistol rounds as the ammunition of choice for sub-machine guns, it has gained a reputation as a powerful armor defeating round that will not overpenetrate. This has lead to the adoption of the round for special forces in hostage rescue or CQC situations.
No weapons implemented in any S.T.A.L.K.E.R. games accept this ammunition. However, it can be assumed that the unimplemented FN P90 would have used this ammunition, since no other weapon in the game has this caliber.
5.7x28mm AP rounds are present in Build 1935's resources, complete with inventory icons and an in-game model. However, the ammunition is not carried by any Stalkers and is not available for trade.
Shadow of ChernobylEdit
Unlike Build 1935, Shadow of Chernobyl does not include inventory icons for this ammunition. The in-game model remains present and the ammunition is still implemented within the game's code. Curiously, the ammunition appears to have stats identical to those of 5.56x45mm AP ammunition.
Shadow of Chernobyl specifies another 5.7x28mm ammo type: 5.7x28mm FMJ. However, in real-world terms the Full Metal Jacketed variant of 5.7x28mm ammo is the Armor Piercing variety. 5.7x28mm FMJ rounds are likely meant to represent some other, non-armor piercing variety of round, such as Jacketed Hollow-Point or Sporting rounds.