In the US it was developed by the Marine Corps during the mid/late 1980s and was approved for service use in 1990 during Operation Desert Storm.
The SLAP design incorporates a polymer sabot, which allows for the use of a tungsten penetrator projectile of a lesser diameter than the original bore.
By using the casing of a large cartridge with a lightweight projectile, the velocity of the projectile is greatly increased and the sectional density is improved without requiring a (potentially dangerous) increase in chamber pressure.
[1] 50 SLAP ammunition is completely interoperable with M2 machine guns with stellite liner.
SLAP ammunition is produced by the Winchester Cartridge Company and Olin Manufacturing.