An AR-57 upper for use with an AR-15 lower uses the same 5.7x28 ammo and 50 round mags as the P90.
They use the AR mag well to eject spent cases. They are lots of fun to shoot.
As with most FN guns, the P90 has outstanding quality and reliability.
No first hand experience with the P90.
These days, as with most, the ammo is expensive, and a little hard to find.