In this latest Deathcore Airsoft video, the King Arms FN P90 gets the chrono and shooting test treatment and here are the findings after going through the test...  "The performance test of the King Arms P90 Airsoft AEG with full trades from FN Herstal; overall, this is a solid performing gun that is in the end crippled by a poorly-feeding magazine."

Now that sucks, perhaps using the Classic Army or Tokyo Marui P90 magazines would do the trick.