I ran a P90 for quite some time. It was a pain to carry enough mags.
this is what my kit looked like. It was more urban warfare oriented. I could only carry 4 mags on my main kit, and maybe 2 extra on a drop leg. Mag changes where a pain. especially those on the drop leg.
The P90 itself work verry well. Even at -40 in winter. But the problem is it's mags. They are too long!