Well its not a popular ammo, as its an odd caliber. But it wasn't easy to find before the rush on guns/ammo.
Always loved that gun though, and the P90. Although I can't stand the look of the civilian model with its 16" barrel though.....
As for the five-seven, soft shooting, fast follow up shots, large capacity mag whats not to love!!
IIRC the 5.7 has some amazing ballistics for such a soft shooting round (not to mention its also used as a pistol round, like yours) military loadings could defeat class IIIa body armor!!
Great gun! Congrats