>>4281245The FN P90 was one of the main two competitors (the other being the HK MP7) for NATO's PDW (Personal Defense Weapon) program. The criteria set by the program entailed a lightweight SMG capable of penetrating kevlar. For this purpose the 5.7mm was specifically designed for use in the P90 and additionally the FiveseveN pistol. IIRC a 5.7 bullet fired from a P90 is capable of penetrating IIIa.