The 5.7X58 was developed in conjunction with the FN P90 for use as a fully automatic personal defense weapon for behind the line support troops. The light recoiling 5.7X58 round was supposed to make it much easier for minimally trained rearguard personnel to use it in full auto. To make it more effective they also developed a true AP round for it, the SS190, which is classified as an armor-piercing handgun round and is regulated as such by the ATF and as
@Rob Boudrie said, is pretty hard to obtain. The FN FiveSeveN was introduced 8 years later to offer a simpler load out for personnel armed with both the P90 (as their in vehicle long gun) and a handgun. With the AP round the 5.7X28, when fired out of the P90, can be effective against level IIIa soft body armor. From the much shorter barreled FiveseveN, even the AP round is much less effective.
Some folks confuse the SS190 AP round with the SS198LF, the “green tip” in a box marked “for military and law enforcement only”. SS198LF uses the same lead free bullet as the SS195LF, but with a slightly higher muzzle velocity (about 100 fps more from the P90). The SS198LF is not regulated by the ATF, since it is not an AP round and the FN “for military and law enforcement only” restriction is largely ignored and the ammo is generally available on the civilian market. The SS198LF is only marginally more effective than the SS195LF when fired from the FiveseveN handgun and only in rare cases have I heard of it actually penetrating body armor. From the longer barreled PS90 the SS198LF does have a decent chance of penetrating IIIa armor, but it is no where near as effective as the SS190 AP.