The P90 is described in the game as:
Light, powerful, and handy in CQC.
It can be picked up in La Cruz province, in the far east in a very large and open cartel base “La Cruz Base”. The weapons case is inside the armoury at the north of the base.
You can drive straight into this open base unchallenged, and although it’s less direct (than entering from the east), following your GPS from the west is very easy. I paused occasionally to take out snipers in their towers, and there’s a drone jammer near the armoury preventing recon with a drone, but you can almost waltz right up to this and just take the gun.
It’s not quite that easy – you may need to do some sneaking, but it’s pretty straightforward.
Like the 5.7 USG pistol – these guns were designed together by FN – this gun also uses the 5.7x28mm round and although I really wanted to prefer the cool Vector .45ACP but I have to say this my favourite SMG. It’s highly modular and has a great balance of a high rate of fire, handling and accuracy, and I really like the 50 round magazine, so there not even a need for a high cap mag.